Is it permissible to act upon a will that dictates the endowment of real estate mortgaged to the bank, on the condition that a portion of its income be spent as an ongoing charity (sadaqah jariyah), a portion as a fee for the trustee (nazir), and the remainder be distributed among the heirs?

The question is incomplete, and matters related to waqf (endowment), its supervisor, and the distribution of its proceeds should be referred to the Sharia judge. It is not permissible to endow a house mortgaged to a real estate bank except with its permission; because an essential mortgaged property cannot be endowed except with the permission of the mortgagee, and it is not permissible for the mortgagor to dispose of it in a way that removes ownership, such as selling, gifting, or endowing, except with the permission of the mortgagee.