Is it obligatory to build a wall around a place designated for a mosque within a single building that includes shops, and is it a condition that the entire building be a mosque, or is it permissible to designate only a part of it?

Firstly: It is permissible to make part of the building into shops or the entire mosque, according to the benefit and the needs of the worshippers. It is preferable to make part of it into shops as an endowment for the mosque, to cover its expenses. It is permissible to have the mosque beneath residential units, provided that the mosque and the residences were built in this manner from the outset.

Secondly: Buying and selling in the mosque is forbidden and invalid, due to the prohibition of the Prophet (peace be upon him) regarding this, and for the hadith: "If you see someone selling or buying in the mosque, say: 'May Allah not make your trade profitable.'" A clear boundary for the mosque must be established, whether by a wall or carpets, to prevent selling inside it, and to make known its sanctity and rulings.