פירוש על קידושין 55:2

How do we know that a person can make another take an oath by extension even if he is not certain about that he owes him money? The answer is by comparison with the sotah oaths. The sotah oath is by its very nature a case of uncertainty. We do not know if the woman committed adultery or not. Nevertheless, she can be forced to take an oath and an oath by extension. So too in cases that occur outside the Temple—one can force another to take an oath by extension even in cases of uncertainty.