Is it permissible not to return the down payment to the buyer who backs out, if the contract stipulates that, and the seller has disposed of the amount and entered into a partnership that prevents him from reselling the property or returning the amount?

This type of transaction is known as a non-refundable deposit sale (bay' al-urboun). The majority of jurists prohibit it due to the presence of gharar (excessive uncertainty), consuming people's wealth unjustly, and the existence of two invalid conditions within it. However, the Hanbalis permitted it, citing that Ahmad (ibn Hanbal) allowed it and that Umar (ibn al-Khattab) practiced it, and that Ibn Umar and Ibn Sirin also permitted it. The Islamic Fiqh Academy has favored the opinion of the Hanbalis, provided that the legitimate Islamic regulations are met. Based on this, the questioner is permitted to own the money if the regulations for its sale are met.