Responsa על בבא בתרא 60:6

Q. When A demanded of B the return of the money he had given him, B repaid part of it and provided A with a surety for the rest. The fact that B provided A with a surety is attested to by witnesses. Now, however, B claims that this money was originally given to him by A not as a loan but as an outright gift, that consequently he owes nothing to A, and that when he provided A with a surety he acted "inside the line of justice."
A. The testimony of the witnesses to the effect that B provided A with a surety, is convincing proof that A had given the money to B as a loan. Although a person will sometimes buy what by law belongs to him in order to avoid litigation, this principle applies only when the rightful owner is not in possession of the disputed property, while in our case B was in possession of the money. Moreover, even if there were no such witnesses, we would have put no credence in B's assertion that he provided A with a surety only because he acted "inside the line of justice." For the principle of Miggo — that we ought to believe B's present statement since, were he inclined to lie, he could have denied altogether having provided A with a surety — does not apply to this case, since we are thoroughly convinced that had B not owed the money to A, he would not have repaid part of it.
SOURCES: Cr. 166; Am II, 164.