What is the ruling on renting out part of the mosque building for commercial purposes and spending the proceeds on the mosque?

If a portion of the mosque is not needed for prayer, it is permissible to rent it out for a permissible purpose that is compatible with the mosque's sanctity, provided that the majority of the mosque's congregants agree, in order to achieve a benefit for the mosque. The Hanbalis permitted raising the mosque and building shops beneath it for this purpose.