You cannot sell something that you do not possess

 Loan and Sale cannot be combined in the same transaction

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said in al-Fatawa al-Kubra (4/39): The prohibition on all these types of trickery [loopholes] is very significant because of the report narrated by ‘Abdullah ibn ‘Umar, according to which the Prophet (blessings and peace of Allah be upon him) said: “It is not permissible to have a loan and a sale in the same transaction, or to stipulate two conditions in one transaction, or to earn a profit on something for which one is not liable, or to sell something that you do not possess.” Narrated by the five imams: Ahmad, Abu Dawud, an-Nasa’i, at-Tirmidhi and Ibn Majah. At-Tirmidhi said: It is hasan sahih. Thus the Prophet (blessings and peace of Allah be upon him) forbade combining a loan and a sale. If a loan is combined with a rental, this is combining a loan and a sale, or is similar to that.

Any favour that is combined with buying or selling or renting – such as gifts, loans, the harvest of a palm tree, favouritism in the case of musaaqaah (when trees are given to someone to tend and irrigate in return for a share of the harvest) and muzaara’ah (where one person supplies land and another cultivates it), and so on – is like a loan.

To sum up the meaning of the hadith: a transaction and a gift should not be combined, because the gift is only given because of the transaction and it is not a pure gift, and thus it becomes part of the transaction deal. But if the two parties agree that it is not part of the transaction deal, then they are combining two things that are completely different. So if someone lends one thousand dirhams to a man and sells him an item that is worth five hundred for one thousand, he has only been content to give him a loan because he bought the item for the increased price, and the purchaser was only content to buy the item at that increased price because of the thousand that he needed to borrow. Hence this transaction is not selling something for one thousand, and it cannot be described as a loan only; rather, in reality, he gave him a loan of one thousand and sold him an item for two thousand.

Complete Answer:

https://islamqa.info/en/answers/326594/ruling-on-making-a-down-payment-to-buy-an-apartment-that-is-under-construction-and-ruling-on-receiving-interest-on-that-payment