Muwatta Imam Malik Book of Business Transactions, Hadith 101

Malik related to me from Yahya ibn Said that he heard Muhammad ibn al-Munkadir say, "Allah loves his slave who is generous when he sells, and generous when he buys, generous when he repays, and generous when he is repaid."
Malik said about a man who bought camels or sheep or dry goods or slaves or any goods without measuring precisely, "There is no buying without measuring precisely in anything which can be counted . "
Malik said about a man who gave a man goods to sell for him and set their price saying, "If you sell them for this price as I have ordered you to do, you will have a dinar (or something which he has specified, which they are both satisfied with), if you do not sell them, you will have nothing," "There is no harm in that when he names a price to sell them at and names a known fee. If he sells the goods, he takes the fee, and if he does not sell them, he has nothing."
Malik said, "This is like saying to another man, 'If you capture my runaway slave or bring my stray camel, you will have such-and-such.' This is from the category of reward, and not from the category of giving a wage. Had it been from the category of giving a wage, it would not be good."
Malik said, "As for a man who is given goods and told that if he sells them he will have a named percentage for every dinar, that is not good because whenever he is a dinar less than the price of the goods, he decreases the due which was named for him. This is an uncertain transaction. He does not know how much he will be given."

وَحَدَّثَنِي مَالِكٌ، عَنْ يَحْيَى بْنِ سَعِيدٍ، أَنَّهُ سَمِعَ مُحَمَّدَ بْنَ الْمُنْكَدِرِ، يَقُولُ أَحَبَّ اللَّهُ عَبْدًا سَمْحًا إِنْ بَاعَ سَمْحًا إِنِ ابْتَاعَ سَمْحًا إِنْ قَضَى سَمْحًا إِنِ اقْتَضَى . قَالَ مَالِكٌ فِي الرَّجُلِ يَشْتَرِي الإِبِلَ أَوِ الْغَنَمَ أَوِ الْبَزَّ أَوِ الرَّقِيقَ أَوْ شَيْئًا مِنَ الْعُرُوضِ جِزَافًا إِنَّهُ لاَ يَكُونُ الْجِزَافُ فِي شَىْءٍ مِمَّا يُعَدُّ عَدًّا . قَالَ مَالِكٌ فِي الرَّجُلِ يُعْطِي الرَّجُلَ السِّلْعَةَ يَبِيعُهَا لَهُ وَقَدْ قَوَّمَهَا صَاحِبُهَا قِيمَةً فَقَالَ إِنْ بِعْتَهَا بِهَذَا الثَّمَنِ الَّذِي أَمَرْتُكَ بِهِ فَلَكَ دِينَارٌ - أَوْ شَىْءٌ يُسَمِّيهِ لَهُ يَتَرَاضَيَانِ عَلَيْهِ - وَإِنْ لَمْ تَبِعْهَا فَلَيْسَ لَكَ شَىْءٌ إِنَّهُ لاَ بَأْسَ بِذَلِكَ إِذَا سَمَّى ثَمَنًا يَبِيعُهَا بِهِ وَسَمَّى أَجْرًا مَعْلُومًا إِذَا بَاعَ أَخَذَهُ وَإِنْ لَمْ يَبِعْ فَلاَ شَىْءَ لَهُ . قَالَ مَالِكٌ وَمِثْلُ ذَلِكَ أَنْ يَقُولَ الرَّجُلُ لِلرَّجُلِ إِنْ قَدَرْتَ عَلَى غُلاَمِي الآبِقِ أَوْ جِئْتَ بِجَمَلِي الشَّارِدِ فَلَكَ كَذَا . فَهَذَا مِنْ بَابِ الْجُعْلِ وَلَيْسَ مِنْ بَابِ الإِجَارَةِ وَلَوْ كَانَ مِنْ بَابِ الإِجَارَةِ لَمْ يَصْلُحْ . قَالَ مَالِكٌ فَأَمَّا الرَّجُلُ يُعْطَى السِّلْعَةَ فَيُقَالُ لَهُ بِعْهَا وَلَكَ كَذَا وَكَذَا فِي كُلِّ دِينَارٍ . لِشَىْءٍ يُسَمِّيهِ فَإِنَّ ذَلِكَ لاَ يَصْلُحُ لأَنَّهُ كُلَّمَا نَقَصَ دِينَارٌ مِنْ ثَمَنِ السِّلْعَةِ نَقَصَ مِنْ حَقِّهِ الَّذِي سَمَّى لَهُ فَهَذَا غَرَرٌ لاَ يَدْرِي كَمْ جَعَلَ لَهُ .