Is working in an economic and social development "fund" - which provides easy loans to members by reducing interest and increasing grace and repayment periods, offers non-refundable aid and grants, distributes its profits among members while 10% goes to expenses and administration - permissible or impermissible?

Lending with interest, whether small or large, is forbidden. It is usury (riba), and there is a consensus on its prohibition. Working in a fund based on usurious lending is forbidden, even if the work itself is in an area not directly connected to usury, because it constitutes aiding in it. As for work directly connected to usury, such as writing or reviewing its contracts, it is forbidden, and the sin of its doer is like that of the one who consumes usury. However, if usurious lending is not the dominant activity of the fund, then it is permissible to work in the permissible activities, provided that one does not aid in usury.