Sunan Ibn Majah 2193
It was narrated from 'Amr bin Shu'aib, from his father, from his grandfather that :
the Prophet (ﷺ) forbade the deal involving earnest money. (Hasan)Abu 'Abdullah said: Earnest money refers to when a man buys an animal for one hundred Dinar, then he gives the seller two Dinar in advance and says: "If I do not buy the animal, then the two Dinar are yours."  And it was said that it refers, and Allah knows best, to when a man buys something, and gives the seller a Dirham or less or more, and says: "If I take it (all well and good), and if I do not, then the Dirham is yours."
حَدَّثَنَا الْفَضْلُ بْنُ يَعْقُوبَ الرُّخَامِيُّ، حَدَّثَنَا حَبِيبُ بْنُ أَبِي حَبِيبٍ أَبُو مُحَمَّدٍ، كَاتِبُ مَالِكِ بْنِ أَنَسٍ حَدَّثَنَا عَبْدُ اللَّهِ بْنُ عَامِرٍ الأَسْلَمِيُّ، عَنْ عَمْرِو بْنِ شُعَيْبٍ، عَنْ أَبِيهِ، عَنْ جَدِّهِ، أَنَّ النَّبِيَّ ـ صلى الله عليه وسلم ـ نَهَى عَنْ بَيْعِ الْعُرْبَانِ ‏.‏ قَالَ أَبُو عَبْدِ اللَّهِ الْعُرْبَانُ أَنْ يَشْتَرِيَ الرَّجُلُ دَابَّةً بِمِائَةِ دِينَارٍ فَيُعْطِيَهُ دِينَارَيْنِ أَرْبُونًا فَيَقُولَ إِنْ لَمْ أَشْتَرِ الدَّابَّةَ فَالدِّينَارَانِ لَكَ ‏.‏