Is a friend entitled to increase the agreed-upon sale price of a house multiple times after the buyer has renovated it and spent money on it, especially since there is no legal document to prove the initial agreement?

If offer and acceptance occur, and an agreement is reached on the price and the item sold, and the gathering disperses, then the sale is concluded and binding on both parties. The transfer of ownership of both substitutes (the item and the price) is established. It is not a condition for the contract to be documented in writing. It is not permissible to oblige the buyer to pay an increase in price after the sale has been concluded. With the transfer of ownership, the buyer owns the purchased item and the seller owns the price, even if the price is deferred. The dispositions of each party regarding what they own become effective. It is not permissible to stipulate that the seller retains ownership of the item sold until the price is paid. If the seller obliges the buyer to pay an increase in price after the sale has been concluded, and the buyer pays it under duress, this money is not lawful for the seller, and he must return it unless the buyer permits it, for the Prophet, peace and blessings be upon him, said: "The property of a Muslim person is not lawful except with his willing consent."