What is the ruling of the Sharia regarding the increase or decrease that occurs in the custody of the treasurer, and is it permissible for him to keep the surplus to cover future deficits?

The surplus funds, if their owners are known, must be returned to them. If their owners are unknown, these funds must be given in charity on behalf of their owners. It is not permissible to use them to cover a deficit. The deficit is the questioner's responsibility if it is due to their negligence. These unknown funds can only be rightfully possessed by spending them on the poor, the needy, and other charitable causes.

Abd al-Rahman ibn Qasim al-Hanbali said: "Pledges, deposits, and similar trusts, as well as illicitly acquired funds like stolen or looted money, if their owners are unknown, should be handed over to the ruler or given in charity on behalf of their owner, provided one guarantees it to them."

Sheikh al-Islam Ibn Taymiyyah said: "Funds whose rightful owner is unknown... It is obligatory for the one who has acquired them to return them to their rightful owner. If that is impossible, then the unknown is like the non-existent... This type [of wealth] is forbidden only because it relates to the right of another. If the other is non-existent, or entirely unknown, or completely beyond reach, then the right related to it is absolutely relinquished... If its destruction is forbidden and withholding it is worse than destroying it, then spending it becomes necessary. It does not have a specific expenditure, so it should be spent on all avenues of righteousness and acts of devotion through which one seeks closeness to Allah."