Muwatta Imam Malik Book of Business Transactions, Hadith 1

Yahya related to me from Malik from a reliable source from Amr ibn Shuayb from his father from his father's father that the Messenger of Allah ﷺ forbade transactions in which nonrefundable deposits were paid.
Malik said, "That is, in our opinion, but Allah knows best, that for instance, a man buys a slave or slave-girl or rents an animal and then says to the person from whom he bought the slave or leased the animal, 'I will give you a dinar or a dirham or whatever on the condition that if I actually take the goods or ride what I have rented from you, then what I have given you already goes towards payment of the goods or hire of the animal. If I do not purchase the goods or hire the animal, then what I have given you is yours without liability on your part.' "
Malik said, "According to the way of doing things with us there is nothing wrong in bartering an arabic speaking merchant slave for abyssinian slaves or any other type that are not his equal in eloquence, trading, shrewdness, and know-how. There is nothing wrong in bartering one slave like this for two or more other slaves with a stated delay in the terms if he is clearly different. If there is no appreciable difference between the slaves, two should not be bartered for one with a stated delay in the terms even if their racial type is different."
Malik said, "There is nothing wrong in selling what has been bought in such a transaction before taking possession of all of it as long as you receive the price for it from some one other than the original owner."
Malik said, "An addition to the price must not be made for a foetus in the womb of its mother when she is sold because that is gharar (an uncertain transaction). It is not known whether the child will be male or female, good-looking or ugly, normal or handicapped, alive or dead. All these things will affect the price."
Malik said that in a transaction where a slave or slave-girl was bought for one hundred dinars with a stated credit period that if the seller regretted the sale there was nothing wrong in him asking the buyer to revoke it for ten dinars which he would pay him immediately or after a period and he would forgo his right to the hundred dinars which he was owed.
Malik said, "However, if the buyer regrets and asks the seller to revoke the sale of a slave or slave-girl in consideration of which he will pay an extra ten dinars immediately or on credit terms, extended beyond the original term, that should not be done. It is disapproved of because it is as if, for instance, the seller is buying the one hundred dinars which is not yet due on a year's credit term before the year expires for a slave-girl and ten dinars to be paid immediately or on credit term longer than the year. This falls into the category of selling gold for gold when delayed terms enter into it."
Malik said that it was not proper for a man to sell a slave-girl to another man for one hundred dinars on credit and then to buy her back for more than the original price or on a credit term longer than the original term for which he sold her. To understand why that was disapproved of in that case, the example of a man who sold a slave-girl on credit and then bought her back on a credit term longer than the original term was looked at. He might have sold her for thirty dinars with a month to pay and then buy her back for sixty dinars with a year or half a year to pay. The outcome would only be that his goods would have returned to him just like they were and the other party would have given him thirty dinars on a month's credit against sixty dinars on a year or half a year's credit. That was not to be done.

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنِ الثِّقَةِ، عِنْدَهُ عَنْ عَمْرِو بْنِ شُعَيْبٍ، عَنْ أَبِيهِ، عَنْ جَدِّهِ، أَنَّ رَسُولَ اللَّهِ ﷺ نَهَى عَنْ بَيْعِ الْعُرْبَانِ .