Selling that which one does not own and that whose ownership has not been accomplished is forbidden due to the ahadith concerning that. The ahadith came generally including all sales of that which is not owned or whose ownership is not completed. He (saw) said:

And he said to Uttab bin Usayd:

However these general evidences have been specified in other than the sale of advance credit. As for advance credit sale, the Shar’a has excluded it from the prohibition and permitted it. He (saw) said:

The “salam” is the “salaf” with two “fatha” in weight and meaning. It is the one who delivers present compensation for a described compensation as surety for a period i.e. advance money as price for a good he will possess after a time for a specific period. The advance credit sale is a category of sale which is contracted according to what a sale is contracted, and by the word “salam” and “salaf”. And it is (also) said “aslam” and “aslaf”. The same conditions are considered as in a sale. The transaction of people in salam and tasleef takes place because they are in need of it particularly the farmers and traders. The owners of crops and fruits need expenditure for themselves and upon it to complete what these crops and fruits require of work. Money could become scarce such that they do not have it, so they sell their produce before it emerges for an advance price which he takes possession of immediately in the contract session (majlis al-‘aqd) upon the condition of delivering the good to the buyer when the imposed period falls due. The trader would sell the goods not with them for a specific period which they would determine, and they take possession of the price immediately in the contract session upon condition of delivering the good when the imposed period becomes due.