Siman 208

A transaction performed in a prohibited manner—for example, where the market price for wheat was a se'ah for a sela, and he paid a sela and a quarter per se'ah because the seller would wait for the money, and this is "a reward for waiting" and constitutes rabbinic ribbit and not Torah ribbit because it is by way of buying and selling and not by way of a loan; and similarly, when he purchased while the market price had not yet been established, and in Yoreh Deah siman 175 it was explained that it is prohibited then to set a price for a future time if the seller does not possess the species he is selling, because we concern ourselves that perhaps the market price will become more expensive than the price he set with him and it will cost the seller dearly, and it turns out that he purchased from him cheaply because of the money he advanced to him, and this is also rabbinic ribbit—nevertheless, the transaction is valid. This is specifically when it was established with a kinyan or with one of the other methods of acquisition, such as in the first case where he pulled the wheat into his domain, and in the second case when he advanced the money and they performed a kinyan; for if not, he does not receive a "He who punished" because of the money, since it was performed in a prohibited manner, and how could we curse him for canceling the sale? On the contrary, may a blessing come upon him. But if it was established with a complete kinyan, the transaction is valid [and according to the Shach in siman 209, subsection 13, it must be said that it was by way of an obligation; and examine this closely]:

And even though neither of them can retract, nevertheless, in a case of one who buys at a high price in exchange for the waiting of the money, he shall not pay according to the rate he established with him, but rather like the market price. And neither of them can void the transaction since there was a fixed price at that time. And so too, if he advanced money to him and the market rate had not yet emerged, if it did not become more expensive than the price they established, the transaction stands and is not subject to retraction. And if the market rate became more expensive than the price they established, if the seller does not wish to reduce from the market rate, the buyer can retract so as not to pay according to the expensive rate, for how can we compel the buyer to receive at a higher price than what he established with him when there was no fixed market price at that time? And also the seller cannot be compelled to give according to the price he established with him, for since the market rate had not emerged, this is not a transaction at all, and also how can we compel him to perform a prohibition? This is not the case regarding one who buys at a high price because of the waiting of the money, where we certainly compel the seller since there was a fixed price at that time. And when the buyer wants to pay according to the expensive rate, certainly the transaction stands. And if he gave him according to the price he established with him, we do not extract it from the buyer, for anything that is not Biblical ribbit is not extracted by the dayanim, as I wrote there in siman 161. And in any manner of buying and selling, there is no Biblical ribbit here. And there is no difference between a purchase and a gift or a waiver, for even if it was done with a prohibition, such as on Shabbat or Yom Tov and the like, nevertheless the matter stands, as I wrote at the end of siman 195.

Our teacher, the Rema, wrote: And likewise, if one swore or vowed not to sell, and transgressed and sold, the sale is valid. Thus far his words. And in Yoreh Deah siman 230, he wrote that some say anyone who swore not to do a certain thing, such as if he swore not to sell a certain object and transgressed and sold it, his sale is nothing, since he transgressed his oath. Thus far his words. And the truth is that there is a dispute of the Rishonim in this matter, and regarding the halachah, the great Acharonim wrote that the sale is void based on the principle: "Any matter that the Merciful One said 'do not do,' if one did it, it is not effective." And the opinion of the Rambam also appears so [Mishneh LaMelech chapter 9 of Malveh]. And even regarding a rabbinic prohibition we say this [Ketubot 81b]. And that which was explained that the sale is valid even with a prohibition, this is specifically when the essence of the sale is not prohibited, but rather it contains a detail of prohibition and he could have done it in a permitted manner, such as if he bought wheat cheaply because he advanced money to the seller, and likewise one who sells before the market price is established, and likewise one who sells on Shabbat, for he could have performed this acquisition on a weekday, or according to the market price, or when there is a market price. But in one who swears not to sell this object, behold, there is no side of permission at all in this sale [Sma]. And some say that it is a legal doubt [Shach and Bach]. And even though he has the ability to request absolution for the oath, this is not called having the ability to do it in a permitted manner, for "new faces have come here" and there is no oath here at all; but as long as the oath remains in its place, there is no side of permission in it. And there is one who says that the primary halachah is that even in this the sale is valid, for the prohibition is not in the sale but in the oath, and it is not called a prohibition in the sale itself unless the sages commanded not to sell this object [like that in Ketubot ibid.], and not when the prohibition is in the oath [Taz]. Furthermore, some say that this entire rule of "if one did it, it is not effective" applies only to prohibitions that the Torah or the sages decreed as a prohibition, and not to a prohibition that a person devised himself and made into a prohibition because of his oath [Tosafot Temurah 6a, s.v. "ve-hashta"]. And some raise difficulties against what we wrote, that in a place where one can do it in a permitted manner we do not say "if one did it, it is not effective," for behold, in all the prohibitions that the Gemara enumerated [ibid.] regarding this matter, it is possible to do them in a permitted manner [Ktzot HaChoshen and Netivot HaMishpat]. However, in a matter where the Torah explicitly commanded "do not do this thing" specifically, and he transgressed and did it, certainly it is not effective since he transgressed a Torah law. This is not the case in sales when he did it with a side of prohibition, for the Torah did not specifically command "do not sell in this manner," but rather the prohibition is included among the prohibitions, and since he could have sold without the prohibition, we do not say "if one did it, it is not effective" [it appears to me]. And some distinguish between a matter that can be retracted to nullify the prohibition, in which case we say "if one did it, it is not effective" in order to remove him from the prohibition he performed, and a matter that cannot be retracted, where the prohibition that was done is done, and there is no benefit if we nullify the matter since the prohibition has already been performed and cannot be restored [Maharit and Netivot HaMishpat]. Therefore, in every sale involving a prohibition, we uphold the sale and nullify the prohibition, such as in a sale of "reward for waiting" and a sale on Shabbat if we nullify it, for the prohibition that was done is done. And according to this, in advancing money before the market price was established and it became expensive, we should have compelled both of them not to uphold the sale according to what they stipulated, but it does not appear so from the poskim. And according to this view, one should be in doubt regarding an oath not to sell whether to nullify the sale or to uphold it and request absolution for his oath. And perhaps this is the intention of our teacher, the Rema, that here he wrote that the sale is valid, and in Yoreh Deah, which deals with the prohibition of an oath, he wrote that the sale is void, meaning until he requests absolution for the oath, but he is obligated to request absolution for the oath in order to uphold the sale. And furthermore, it is possible to say that certainly the buyer has the ability to uphold the sale against the will of the seller, for what does he have to do with the seller's oath? But if the buyer is not concerned with its nullification, we compel the seller to nullify the sale so that he does not transgress his oath [and therefore he was precise to say "the sale is valid" and there he wrote "his sale is nothing"]. And the primary halachah is like his words here [and there is clear proof from the Mishnah in Shevuot 25a: "an oath that I will not give," and it considers it an oath of expression; and if it enters your mind that his giving is nothing, how do you find that he transgresses his oath? For even if he gave it and the recipient did not return it, it is like theft in his hand].

In Yoreh Deah siman 161 it was explained that ribbit ketzutzah is recoverable by the beit din, and nevertheless, some say that as long as the ribbit is in the hand of the lender, the money is considered his and he may acquire items with it and betroth a woman, only that he must return this sum because of the Torah's decree "that your brother may live with you." And the proof for this is that since the heirs are exempt from paying, as is written there, certainly the money is his, but the Torah obligated him to return such a sum to the borrower [Netivot HaMishpat in the name of the Ritva]: