What is the ruling on supplicating after the obligatory prayer, and on departing before the Imam in prayer? Is it permissible to utilize the old mosque and convert it into a residence after a new mosque has been built next to it?

Supplication (Du'a) after the obligatory prayer is divided into two types: The first consists of supplications mentioned in the Sunnah, such as prayers for forgiveness and others. The second consists of supplications not mentioned [in the Sunnah]; this is permissible, but it should not be collective nor with raised hands. Some scholars consider the consistent practice of supplication after the obligatory prayer an innovation (bid'ah). As for departing before the Imam, it is forbidden before he concludes the prayer with the salutations (taslim). It is recommended not to depart before the Imam turns away from the direction of the Qiblah. Regarding benefiting from the old mosque, a mosque is an endowment (waqf) and it is not permissible to utilize it except for the purpose for which it was endowed, unless there is a benefit in replacing it. In that case, it is permissible to utilize the old mosque for purposes other than that for which it was endowed. The one who considers this matter is the supervisor (nazir) or the Ministry of Awqaf.