Narrated 'Amr b. Suh'aib: 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.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ، قَالَ قَرَأْتُ عَلَى مَالِكِ بْنِ أَنَسٍ أَنَّهُ بَلَغَهُ عَنْ عَمْرِو بْنِ شُعَيْبٍ، عَنْ أَبِيهِ، عَنْ جَدِّهِ، أَنَّهُ قَالَ نَهَى رَسُولُ اللَّهِ صلى الله عليه وسلم 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 .
sunnah.com