RSRH; One other point. On a general fund Pushka (Not designated to a specific Tzedakah) if you need the funds on an urgent basis and have no other funds at this time, even if funds are available to you on a later date, you are not “M’chuyav” (Al Pi Halacha) to return the funds. Because you are considered an “Ani” (Poor) at this time. This is a Halacha in Hilchos Tzedakah.
Another point, on the OP’s question; “Ribbis” (Charging interest) is permitted for a Tzedakah fund. BTW. A manager/trustee of an orphans’ fund is allowed to collect interest, too.