Siman 21

The litigants have the ability to stipulate between themselves any conditions they desire. For example, if one of the litigants was obligated in an oath, and when he swears he will take money from his fellow, or when he swears he will be exempt from paying, he can stipulate that "if I do not come on such-and-such a day to swear, my fellow litigant shall be believed in his claims without an oath," whether to take or to be exempt, for any condition regarding monetary matters is valid. And when the time passed and he did not come, his fellow is immediately exempt from paying, or he takes money from him, except that they give him time for payment as written in siman 100 and as written in siman 19; however, the obligation or the exemption takes effect immediately. And so too in the case of other conditions that they stipulate with one another.

However, any condition she makes is not effective when they stipulated with mere words, and it requires a kinyan and in a manner that it should not be like an asmakhta according to what will be explained in siman 207. And even regarding the matter that his fellow shall be exempt from it, which is like "it is forgiven to you," and a mechila does not require a kinyan as is written in siman 12, nevertheless here it requires a kinyan since it is by means of a condition and is close to a mechila in error, and in siman 207 it will be further explained.

And if he brought proof that he was under duress, even if the duress was only on the final day of the time period, he has not lost his right, for the Merciful One exempts one under duress, and the law returns to as if there had been no condition at all. And even if after the time period the duress passed, the condition is void, because immediately at the end of the fixed time when he was under duress, the condition became void. However, the duress must be a matter that is somewhat uncommon, for if it were a duress that is very common, he is not called "under duress," as he should have stipulated that such a duress would void the condition; for he should have considered it since it is common, and since he did not stipulate, he is the one who caused himself the loss [Shach siman 55]. But regarding duress that is not very common, he did not have to consider it and did not need to stipulate. And all this is when he brought proof that he was under duress, but without proof, he is not believed to say that he was under duress even when he is the possessor [Urim VeTummim], for possession is not effective for uncommon duress, as without proof we are like witnesses that there was no duress, and possession in the place of witnesses is nothing.

That which we have said, that an unavoidable accident cancels the condition, this is specifically for one who obligated himself that if he does not do this thing by such-and-such a time, and an unavoidable accident occurred to him and he did not do it, he is exempt. However, if he obligated himself such-and-such when his fellow does a certain thing by such-and-such a time, and his fellow was prevented by an unavoidable accident and did not do the thing, we do not say that since he was prevented by an unavoidable accident and the Merciful One exempted him, this one who made the condition should become obligated. For this is no logic at all; for should this one become obligated because of the unavoidable accident of that one? What has he to do with the unavoidable accident of his fellow? For while it is true the Torah exempted one under an unavoidable accident, the Torah did not obligate the other because of the unavoidable accident of this one. And since the other did not fulfill the condition, for whatever reason it may be, it follows that he is not obligated in what he conditioned [Shach].