Siman 234

One who sells a food item to his fellow and afterwards it becomes known that it is a forbidden item, whether it is a Torah prohibition or a rabbinic prohibition, even if it is only due to the stringency of the poskim, the sale is void and he must return the money to him, for you have no greater defect than this. And if the buyer already ate it, if it is a Torah prohibition, he returns the money to him, and what he ate, he ate. The reason is because they penalized the seller who sold a forbidden item [Rashi at the end of the fifth chapter of Bechorot, and Nimukei Yosef the sixth chapter of Bava Batra, and Levush, and Shach Yoreh Deah siman 119, subsection 25]. According to this reason, it is only when the seller was like a willful transgressor in a matter where he could have known of the prohibition; but if he was completely unintentional, such as if he sold meat and afterwards a treifah was found in some organ, and according to the law one is not required to check for all treifot except for the lung, as written in Yoreh Deah at the beginning of siman 39, he does not need to return the money to him when he ate it. However, some say another reason: because regarding a Torah prohibition, when an Israelite ate it, it is not considered a benefit at all, and on the contrary, it is a great distress to him. According to this reason, in any case the seller must return the money to him. There is also a practical difference in the reverse: if the buyer nullified it in sixty in a matter where the prohibition is not recognizable and it turned into being like permitted food, according to the second reason he does not need to return the money, and according to the first reason he must [Netivot HaMishpat]. The main point is like the first reason, for so wrote the Rishonim.

But if it was a rabbinic prohibition and he ate it, the seller does not return anything to him. And even though once it was revealed that it was a prohibition, the purchase is not worth its price, and why should he not return the excess over its value? Nevertheless, since he ate it and did not know it was a prohibition, it is considered for the buyer like a permitted item, for he did not commit a transgression, and it is worth the money he gave. And some say that this is specifically when he already paid the money, but when he has not yet paid, he shall only pay as for a treif [Chatam Sofer]. And so it appears, for how can we obligate him to pay initially as for a kosher item for that which he already ate? And all this is regarding prohibitions of eating, but regarding prohibitions of benefit, even if they are rabbinic, he returns the money even when he ate it, for they do not have the status of a sale at all. And behold, they have lost nothing, for they are prohibited for benefit. And even when the buyer sold it to a gentile and received money for this, behold the money is prohibited, for there are prohibitions of benefit that seize their value, such as idolatry and wine of libation. And even regarding prohibitions of benefit that do not seize their value, such as orlah and kilayim of the vineyard, this is for others, but the seller himself is prohibited from benefiting from their money, as was written in Orach Chaim siman 450 regarding chametz after Pesach, and therefore he must return all the money to the buyer [Sma]:

Therefore, one who slaughters a firstborn on account of its blemish and sold it, and it became known that he did not show it to an expert or to three members of the synagogue, such that it is forbidden for benefit by rabbinic decree as is explained in Yoreh Deah siman 310, what they ate, they ate, and he must return the money to them. Regarding what remains of the meat in the hands of the purchasers, he himself shall bury it, and he shall not give it to the seller to bury it, for he is not to be trusted, lest he sell it to a gentile. The seller returns the money to him. So too, one who slaughters a cow and sold it, and it became known that it is a terefah by Torah law, what they ate, they ate, and he must return the money to them. Regarding what they did not eat, they shall return the meat to him so that he may sell it to a gentile, and he shall return the money to them. And if you ask, how can we return the meat to him, let us be concerned lest he sell it to an Israelite? It can be said that since the matter has become public, he will not do this and they will not buy from him. Furthermore, one who sells a forbidden item intentionally, we penalize him so that he shall not sell anymore, as is written in Yoreh Deah siman 119. If the purchasers sold it to a gentile or fed it to dogs, they shall calculate with the butcher how much is due to him for the value of a terefah, and the butcher shall return the remainder to them. If they have not yet paid money and wish to withhold the meat in order to sell it to a gentile, they shall pay as for a terefah. Such is the law for anyone who sells an item whose prohibition of eating is from the Torah. If he sold from an animal whose lung was not checked properly, it is like a rabbinic prohibition, for the checking is by rabbinic decree.

If the buyer mixed the forbidden item with the permitted item in a manner that the whole mixture became forbidden, if the seller sold it intentionally, he must pay him for the entire mixture. This is much more severe than a judge who took and gave with his own hand, who is liable to pay as is written in siman 25; and all the more so this wicked person, who is liable to pay for the mixture and for the vessels if they became forbidden [Netivot HaMishpat]. And if he was unintentional, he does not need to pay even to fulfill his obligation before Heaven [it appears to me]: