Is it permissible to lend endowment funds designated for the mosque to someone in need, with a guarantee of their return, given that the mosque does not currently need them and it is impossible to refer back to the endowers?

It is not permissible for the supervisor of an endowment (waqf) to lend anyone from the endowment's funds. Jurists have stipulated this because it is a condition for the lender that his act of donation be valid. Thus, neither the guardian of an orphan may lend from the orphan's money, nor may the supervisor of an endowment lend from the endowment's money, unless he fears the spoilage or loss of the produce, in which case he may lend it to a trustworthy and solvent person.