Is it permissible for an installment seller to take a portion of the price of the tricycle as a down payment before purchasing it for the buyer, then add interest to the remainder and collect it in installments, or must he first pay the full price of the tricycle before taking the down payment?

If your friend buys the commodity and owns it, then there is no harm in him selling it to you at that point, based on the statement of the Permanent Committee: "If a person asks another to buy a specific car, for example, or one described with a clear description, and promises him that he will buy it from him, and the one who was asked buys it and takes possession of it, it is permissible for the one who requested it to buy it from him afterwards, either in cash or in deferred installments with a known profit."

It is not permissible for him to take the seven thousand [currency units] from you in advance to pay part of the price of the commodity, as this falls under selling what one does not own and profiting from what one does not guarantee, which the Lawgiver has prohibited. This is based on the hadith of Hakim ibn Hizam, who said: "O Messenger of Allah, a man comes to me asking me to sell him something that I do not possess, then I buy it from the market." He said: "Do not sell what you do not possess." Rather, a promise from you to buy is sufficient. What is called a "earnest money deposit" serves as a pledge with the second party, solely as a sign of the sincerity of the buyer, and the second party is not permitted to dispose of it or consider it an advance installment. If he buys the commodity and takes possession of it, then sells it to you, it is then permissible to pay the earnest money deposit as an advance installment of the price.