Responsa for Nedarim 66:2

רבא אמר אפילו תימא דברי הכל גבי מודר הנאה דיהיב על מנת שלא לפרוע

Raba said: You may even say that it agrees with all: [We suppose that] the man who is interdicted by vow not to benefit from his neighbour was lent [money] without obligation to repay.<span class="x" onmousemove="('comment',' The creditor having lent it to be repaid at the debtor's leisure (Ran). Therefore, when his neighbour repays his debt, he confers no benefit upon him. Similarly, he may pay his shekel only when he is not bound to pay it himself, e.g., if he had already sent it and it was lost on the road. ');">²</span>

Maharach Or Zarua Responsa