Is it permissible to take an extra 50 Riyals above the specified selling price? Is selling an apartment to one's wife permissible? Is taking the extra amount as commission permissible?

Firstly: It is not permissible for you to profit from selling your sister's husband's property except with his knowledge, because you are a trusted agent and not a broker. It is forbidden to lie that you will give the profit to a broker unless that is true.

Secondly: If your wife paid the down payment as a promise to buy, then you are obligated to fulfill the promise and sell her the apartment. However, if the down payment accompanied a firm sales contract, then the apartment has become her property, and she is entitled to pay the remaining price. If she does not complete the payment, you have the right to keep the down payment. You were correct in giving the apartment to your wife if she promised to buy; however, if she has already purchased it, you have no option but to give her the apartment.

Thirdly: It is permissible for your wife to sell the apartment to an external buyer, provided that the buyer knows that your wife is the seller, in order to determine the rights resulting from the sale. Ibn Uthaymeen mentioned that if a commodity is sold and delivered, the buyer may sell it in its place. Matters should be conducted clearly from the outset to avoid doubts and to ensure that the principal is aware of the situation. As an agent, you may sell the apartment to your wife, with the stipulation that she is the seller to the external buyer.