Is it permissible to borrow from or dispose of surplus funds that appear in the safekeeping of the treasurer, and what is the ruling on disposing of them if it occurs?

The hand of a treasurer is a hand of trust (amanah), not a hand of guarantee (dhaman). He is not required to compensate for a deficit in accounts unless there is negligence or dereliction of duty on his part. As for an increase in the account, it belongs to it and should only be spent on its designated expenditures. It is not permissible to dispose of it through borrowing or otherwise, except by order of the responsible authority, otherwise, it would be an encroachment on the property of others.