Is it permissible to use the remaining amount from a donation for building a mosque as charity for orphans, or must it be spent on another mosque?

If there is any money left from what a person donated to complete the construction of a mosque, and he appointed you as his agent for that, then it is obligatory to return it to him or to seek his permission regarding how this money should be spent, because it is his property. Therefore, you must inform him of the remaining amount, and he has the choice to take it or to spend it on charitable causes such as alms for orphans or building another mosque.