Is it permissible to borrow from a charitable endowment fund that exceeds its needs, by calculating the cost of apartments built from the surplus revenue of the charitable endowment, then setting their value at a multiplied price and offering them to those interested in contributing to the endowment, so that the contributor pays the multiplied amount and the apartment becomes a general charitable endowment in their name under the management of the institution, and then the multiplied amount is returned to the endowment? And does returning the multiplied loan fall under the category of a loan that brings benefit?

It is permissible for one waqf (endowment) to borrow from another waqf that is self-sufficient from its income or has a surplus thereof. If the self-sufficient waqf is endowed for a specific purpose, then it is permissible through a loan (salaf). If it is a general, unrestricted waqf, then it is permissible to take from it even without it being a loan. It is not permissible to repay a loan with an increase if it was stipulated, because that is usury (riba), and usury applies to waqf funds like any other. In fact, it is not permissible even if the increase was not stipulated, because the waqf administrator may only dispose of waqf funds in a way that benefits the waqf, similar to the guardian and custodian of an orphan's money.