What is the ruling on money remaining from the purchase of an apartment for a sum exceeding its real value, and is expiation for this sin required?

It is not permissible for the client and the original seller or marketer to collude in a Murabaha sale to increase the price so that the client takes the excess, because that leads to usury (riba). The aspect of usury here is that the bank or real estate entity places the profit on the total price, thus resulting in the client having taken a sum of money on the condition that he repays it with an increase. The client must return this amount to the real estate entity, either by informing them of the truth or by claiming early payment of part of what is due. If he is unable to return it, we hope that he will not incur sin due to his ignorance of the ruling, and he may benefit from the money.