Firstly: Selling the prepaid commodity before taking possession of it. [2897] I.e., If he was to pay an advance for rice or wheat and prior from delivery of it from the buyer, he intends to sell it.
The scholars have differed with regard to the ruling of selling the prepaid commodity before taking possession of it:

The first position: It is not permissible to sell the prepaid commodity before taking possession of it under any circumstances, and this is the position of the majority: [2898] The Malikis have specified the prohibition to food, see: ((al-Taj al-Ikleel)) by Mawwaaq (4/542). The Hanafis, [2899] ((Mukhtasar al-Quduri)) (p.88), ((al-Mabsuth)) by Sarakhsi (12/29). Shafi’is, [2900] ((Minhaaj at-Taalibeen)) by Nawawi (p.103), ((Mugni al-Muhtaaj)) by al-Shirbini (2/70). Hanbalis, [2901] ((Sharh Munthaha al-Iraadaat)) by Buhuti (2/96), ((Mataalib Awlayin Nahaa)) by al-Ruhaybaani (3/229). and consensus has been stated on the matter, [2902] Ibn Quddamah said: (As for selling the prepaid commodity prior to delivery of it, we do not know of any dispute regarding its prohibition). ((al-Mugni)) (4/227). And al-Ayni has said: (It is not permissible to carry out transactions with the capital of the ‘salam’ nor the commodity prior to delivery”, and this is by agreement of the jurists). ((al-Binaayah)) (8/356). and this is because it is selling that which has not been taken possession of, and also it is selling a debt that is owed upon someone else, and its seller is unable to hand it over. [2903] See: ((Sharh Mukhtasar al-Tahawi)) by al-Jassas (3/133).

The second position: It is permissible to sell the prepaid commodity before taking possession of it to the person whom it is upon, this upon the condition that it be for a similar price or less, and this is one opinion of Ahmed, [2904] ((Al-Insaaf)) by Mardawi (5/108) and see: ((Majmu al-Fataawah)) by ibn Taymiyyah (29/505). and that of Ibn Taymiyyah, [2905] Ibn Taymiyyah said: (Ahmed was asked about a man that sold some food by way of ‘salam’ for an appointed time, and when the time came, he bought from him a house. So, he said: Yes, he can buy from him that which is not measured or weighed. Also, Harb al- Kirmaani said: I asked Ahmed that if a man pays another man dirhams in advance for wheat, but at the appointed time he does not have the produce, some have said: Barley has been exchanged for the dirhams, so take from the barley. He said: Do not take from him the barley except for a similar measurement of wheat, or less. I said: If the wheat was ten bags, does he take ten bags of barley? He said: Yes. And others have also reported similar to this, and this opinion is mainly from the reports of Ahmed, and it is closest to his principles, because the cause of impermissibility in selling the debt of ‘salam’ is that it is a commodity, so it cannot be sold before delivery, and Ahmed does not take by the opinion of the impermissibility of selling prior to delivery, rather he has nuances to his position that are well known, and for this reason he distinguishes between selling from the seller and selling from others. ((Majmu al-Fataawah)) by ibn Taymiyyah (29/505) and see: ((al-Insaaf)) by Mardawi (5/108). ibn al-Qayyim, [2906] Ibn Qayyim said: (If it is permissible to sell goods to the seller before delivery, then a fortiori using the ‘salam’ as a debt is permissible, just as it is permissible to cancel the transaction before delivery as agreed by all scholars, as opposed to cancellation of goods, and what further clarifies this is that ibn Abbas did not permit the selling of the purchased item prior to its delivery, and the evidence for this position is that the Prophet did not permit the selling of food prior to its delivery, and he said: I take everything to be the same as food, whilst also it is established that he permitted the selling of the debt of ‘salam’ to the person the debt was on as long as this did not result in usuary, despite this, he did not differentiate between food and other things, and neither between that which is measured and that which is weighed and other things). ((Hashiyah ibn al-Qayyim)) (9/258). and ibn Uthaymeen [2907] Ibn Uthaymeen permitted it if he sold it to the original seller, and this is with the following conditions: Firstly: That he does not take a profit by selling for that day’s rate. Secondly: That the exchange happens before they separate if he is selling it for something that would constitute ribaa nasi’ah. Thirdly: That he does not make it the price of another ‘salam’ contract because if he does, it is likely he will profit from it. Ibn Uthaymeen said: (If a person says: Then is it permissible to sell the prepaid commodity prior to its delivery? Then the answer is: Yes, it is permissible to sell it to the original seller, and according to Shaykhul Islam it is permissible to sell it to anyone, but there is an issue to this as realistically if you were to sell it to anyone else other than the original seller then they may be unable to receive it, and also, if you were to sell it to other than the buyer for what it is to be sold as a debt then leeway on this matter is not very clear to me, and Shaykhul Islam had permitted the selling of a debt to other than the debtor but stipulated that he must be able to receive it. If he does sell it to other than the original seller, then there are three conditions to this: First condition: That he does not take a profit by selling it for that day’s rate, because if he sells it for more than that day’s rate then he has profited from that which he is not held liable for, and the Prophet ? prohibited profiting from that which one is not held liable for. For example: He pays an advance for one hundred saa’ of barley, and at the time of the transaction the value of it was only two hundred dirhams, and then he says: I will sell it to you for two hundred and fifty dirhams, then this is not permissible as he has profited from the barley before it has become his responsibility as he does not own it, nor has he taken possession of it, so therefore he has profited from that which he is not held liable for, and also because of the hadith of ibn Umar (may Allah be pleased with him), wherein he stated: ((There is no harm that you take it for its day rate as long as you do not profit from that which you are not liable for)). So, if one hundred saa’ is equal to two hundred dirhams, and he sells it to him for one hundred and fifty then it is permissible, because if it is permissible for the day rate, then a fortiori it is permissible for less than it. Also, we have made the preventing increase as the cause for not being permitted to profit on that which one is not liable for, and in this case, he has not profited but rather made a loss. And the reason for him saying (the day’s rate) is so that it does not increase, and in turn if you decrease then you have done good. Second condition: That the exchange happens before they separate if he is selling it for something that would constitute ribaa nasi’ah, for example: He sells barley for wheat, one hundred saa’ of barley for two hundred saa’ of wheat, this is permissible as long as the exchange happens before the two parties separate, because it is a condition for selling barley in exchange for wheat that the exchange happens before they separate, and because of the hadith of ibn Umar (may Allah be pleased with him), wherein he stated: ((There is no harm that you take it for its day rate so long as you do not separate leaving something still to be settled)); because he has sold dinaars for the dirhams, and a condition for this is that the exchange happens before they separate. Third condition: That he does not make it the price for another ‘salam’ contract, because if he does it is most likely that he will make a profit on it, and as such it becomes profit on something he is not liable for, for example: A ‘salam’ contract was agreed for one hundred saa’ of wheat, then they say: We will make it a ‘salam’ for five sheep, because ‘salam’ in animals is permissible as has been mentioned before, and the five sheep will be of such and such description, to be given after a year then this is not permissible, because most of the time this only done with profit, and these five sheep are equal to one hundred and twenty saa’, and also this leads to changing the debt, in that every time the debt is agreed he makes it into another ‘salam’ contract, and this is trickery by way of changing the debt and increasing at the expense of the debtor through this method, and so every time the debt is agreed he says: Make it another ‘salam’ contract, and like this until the debts accumulate on him. So, the preferable opinion is that the transaction is permissible, but only if the three conditions have been met. ((al-Sharh al-Mumti)) (9/87). too.