Is it permissible to spend from mosque funds to litigate in court over a piece of land endowed by someone for graves, which his heirs now want to reclaim?

If the shops are an endowment (waqf) for the mosque, then their revenue may not be spent except for the benefit of the mosque for which they were endowed. The condition of the endower (waqif) must be observed. If the revenue exceeds the needs of the mosque, it is permissible to spend the surplus for the benefit of a similar endowment, such as another mosque in need. It is also permissible for those in charge of the mosque to spend a portion of the revenue to reclaim the endowed land from those who trespass upon it, if the benefit necessitates it, as some scholars have explicitly permitted changing the condition of the endower for a considered benefit.