[If] one says to another: “I robbed you,” “You lent me,” [or] “You deposited with me,” ... but I do not know whether I returned [it] to you or whether I did not return [it] to you,“ he is obligated to pay. But if he says to him: “I do not know whether I robbed you,” “whether you lent me,” [or] “whether you deposited with me,” he is exempt from paying.