Is it permissible to give money from the inheritance of the deceased paternal uncle as charity to the needy imam of the mosque, as an increase to his modest salary which is insufficient to meet his needs, or to cover his studies and his family's requirements?

This money is no longer owned by your uncle after his death; rather, it has transferred to his heirs, who have the right to dispose of it. If any of the heirs, whether his wife or anyone else, wishes to donate some of this money as charity on behalf of the deceased, it is permissible to do so, and the reward for the charity will reach the deceased, by scholarly consensus. If his wife designates you to give charity from her money on his behalf, it is permissible, and the reward will reach the deceased. There is no objection to paying the charity in installments to the Imam if he is deserving, provided that permission is sought from the donor for the installment payment.