Shevuot 44B

ורב יוסף אמר: כשומר שכר דמי.

and Rav Yosef says that it is like that of a paid bailee, as while performing the mitzva of taking care of a lost item he is exempt from performing other mitzvot that involve the outlay of money, and the money that he saves is tantamount to payment received. Similarly, since safeguarding collateral is part of the mitzva to lend money to the needy, one who does so is exempt during that time from performing other mitzvot that involve the outlay of money. Therefore, R' Eliezer’s ruling, that even if the collateral was lost the creditor collects the entire debt, is in accordance with the opinion of Rabba, and R' Akiva’s ruling, that the creditor loses the value of the collateral, is in accordance with the opinion of Rav Yosef.

לימא דרב יוסף תנאי היא? לא; בשומר אבידה ⁠– דכולי עלמא אית להו דרב יוסף; והכא במלוה צריך למשכון קא מיפלגי: מר סבר מצוה קא עביד, ומר סבר לאו מצוה קא עביד.

The Talmud asks: If so, should we say that the opinion of Rav Yosef is the subject of a dispute between tanna’im? The Talmud rejects this suggestion: No, with regard to one who is safeguarding a lost item, everyone accepts the opinion of Rav Yosef. And here they disagree with regard to a case where the creditor needs the collateral and uses it with the permission of the debtor, deducting a usage fee from the debt. One Sage, R' Akiva, holds that he is still performing a mitzva by safeguarding it, and is therefore exempt from performing other mitzvot during that time. Consequently, he has the status of a paid bailee. And one Sage, R' Eliezer, holds that since he derives benefit from the collateral, he is safeguarding it for his own benefit, and is not performing a mitzva. Therefore, he has the status of an unpaid bailee.