Is it permissible to give the seller the value of the mortgage to release the real estate mortgage on the house, or to give him a down payment on the house's value, before Alinma Bank buys the house and sells it to me?

It is not permissible to give the mortgage value to the homeowner from whom you wish to purchase a house via Murabaha, as this entails one of two things: Either it is a payment for a portion of the house's price, followed by its sale to the bank, and then its resale to you at an increase, which constitutes usury (riba). Or, it is a loan conditioned upon the house being sold to the bank, and then sold to you, which also constitutes usury due to the condition of a benefit in the loan.

It is permissible, however, to purchase a common share of the house and then request the bank to purchase the remaining portion of the house from its owner for you, and then sell it to you via Murabaha.