It would seem that Perutah d'Rav Yosef only applies to a Mitzvah such as watching an Aveidah, which one is obligated to do against his will. It does not apply to a Mitzvah that one voluntarily takes upon himself, such as agreeing to be a Shomer Chinam. Accepting to do a voluntary Mitzvah cannot exempt one from an obligatory Mitzvah, such as giving Tzedakah.

One might question this assumption based on the Gemara (Basa Metzia 82b) which tells us that a Shomer Mashkon also benefits from Perutah d'Rav Yosef, even though it would seem that he has no obligation to lend out money (and to take the Mashkon in exchange). This would imply that even a voluntary obligation of Shemirah benefits from Perutah d'Rav Yosef.

However, lending money is not the same as watching an object as a Shomer Chinam. There indeed is an obligation to lend money to someone who is in dire need of it (if one can afford to lend the money), but there is never an obligation to watch someone's object for him. Watching someone's object involves accepting responsibilities upon one's self, and the Torah does not obligate a person to accept responsibilities in order to help one's friend.

D. Zupnik