On his father's authority, said that his grandfather told that the Messenger of Allah ﷺ forbade the type of transactions in which earnest money was paid. Malik said: This means, as we think--Allah better knows-that a man buys a slave or hires an animal, and he says: I give you a dinar on condition that if I give up the transaction or hire, what I gave you is yours.  

أَنَّهُ قَالَ نَهَى رَسُولُ اللَّهِ ﷺ عَنْ بَيْعِ الْعُرْبَانِ قَالَ مَالِكٌ وَذَلِكَ فِيمَا نَرَى وَاللَّهُ أَعْلَمُ أَنْ يَشْتَرِيَ الرَّجُلُ الْعَبْدَ أَوْ يَتَكَارَى الدَّابَّةَ ثُمَّ يَقُولُ أُعْطِيكَ دِينَارًا عَلَى أَنِّي إِنْ تَرَكْتُ السِّلْعَةَ أَوِ الْكِرَاءَ فَمَا أَعْطَيْتُكَ لَكَ