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If your family collects Tomchei Shabbos from the neighbors, are you allowed to borrow that money?
On one hand the Shulchan Aruch Y:D 259-1 says that after the money was already given to the tzedakah and got to the hands of the gabbai that it may not be borrowed by anyone. However here are numerous poskim who say that since the custom is that the gabbaim do borrow from the money, that it is permitted as if it was done on such a condition. You can rely on the lenient opinion, but it shouldn’t cause a delay in the poor people getting their money. One important practical point is that you make sure to write a note of how much you owe to the tzedakah, and keep it with the money, so you will remember to pay it back.
Pischei Teshuva Y:D 259-4, Shut Chasam Sofer Y:D 110, Derech Emunah Hilchos Matnas Aniyimn 8-25, and footnote 123 in the name of the Chazon Ish. Borach Tzedakah 17 ftnt 16 in the name of R’ S. Z. Auerbach. Tzedkah Umishpat 8-8 brings both opinions.