Is it permissible to take the remaining money from building a mosque in America and spend it on building a library, a school, and annexes for another mosque in a different area that has a large Muslim community?

If the mosque for which the money was collected is completed and no longer needs the funds, then the surplus money should be spent on constructing other mosques and their associated facilities, such as libraries and restrooms, because it is of the same nature as the mosque for which the donors contributed. If there is no mosque in need, then the surplus should be spent on general Muslim welfare projects, such as schools, hospices, and charities for the poor.