Is it permissible to sell an apartment to the bank at a price higher than requested, then for the seller to return the difference to the buyer in cash?

It is not permissible for the original seller and the customer in a Murabaha sale to collude on increasing the price so that the customer obtains additional liquidity; because that leads to usury (riba), as the bank adds profit to the total price, thus the customer would have taken an amount with an increase. The sin falls upon the customer, the assisting seller, and the bank employee if he is aware. This must be refrained from, and the bank should be informed of the approval of the original apartment price, which may prompt the customer to agree to a lower price to reduce the bank's profit.