Is it permissible for a charitable organization that receives various zakat funds from different harvests and at different times to distribute them consecutively throughout the year without the ability to distinguish between the disbursed and remaining funds, and is it permissible for it to keep a permanent amount in its صندوق (cashbox/fund) to cover emergency surgical operations for the poor?

Zakat (definition) on wealth becomes obligatory immediately upon its due date. It is not permissible to delay its delivery to the eligible recipients except for the time reasonably needed for its conveyance. A person may distribute it himself or appoint a trustworthy charity to distribute it immediately to those eligible. It is not permissible to hoard Zakat when there are eligible recipients, unless there is someone in more dire need. As for directly spending Zakat on surgical operations, it is not permissible according to the majority of scholars. However, giving it to the poor and needy for them to perform operations is permissible.