Aruch HaShulchan, Laws of Tzedakah, 251:15

Let us clarify matters that were mentioned previously regarding how to conduct affairs pertaining to tzedakah. This is regarding fixed tzedakah, that each person gives according to their worth and the generosity of their heart. Here we refer to one who takes a fixed amount for tzedakah and does not elaborate which specific area of tzedakah the money should go to. Such as when, prior to death, one specified an amount for the general tzedakah fund, or even not before death, one’s heart guided them to give a fixed amount, and gave the money to the tzedakah collectors of the city. In this case, neither the giver nor their inheritors have any power to allocate the money (to a specific poor person or cause), and this is true also if the giver has only declared their intentions to give but the funds have not yet reached the tzedakah collector. In these cases, the community will do (with the money) what is good in the eyes of God and humanity, according to seven upstanding citizens of the city or according to the majority opinion, or the opinion of the court. [Translation by AJWS]