He—may Allah have mercy on him—was asked:

A man acknowledged in a written deed that he owed another man a sum in dirhams. Some time later, the person in whose favor the acknowledgment had been made came to the witnesses to the deed and said: “The acknowledgment he made is invalid. My only claim against him is for gold belonging to my daughter.” Does this render the acknowledgment void? May a witness testify to the contents of the deed after hearing the creditor say this?

He replied:

The witness testifies to what he heard from the man who made the acknowledgment; nothing more is required of him, whether the creditor confirms or contradicts it.

However, if the creditor makes such a statement, and his words are explained in a way that is ordinarily plausible—for example, by his saying, “My claim against him was for gold, and we agreed that he would acknowledge silver in its place”—then, if the man who made the acknowledgment confirms his account, effect is to be given accordingly. If he denies it, he is to swear an oath denying what the creditor has alleged. And Allah knows best.