Talmud (Rodkinson Edition) — Book 2…
“ If a man gathered money little by little, with the express intention of paying his shekalim tax out of such money, and separated it from other moneys, any remainder which he may have left over after such payment is, according to the school of Shamai, to be devoted for a voluntary offering, because it was separated; and according to the school of Hillel, it is ordinary money, that may be used at will, because it was gathered only for the purpose of paying the amount due, which was already paid. ”
