المعنى الاصطلاحي :

Selling something before having full possession of it and authority to dispose of it.

الشرح المختصر :

"Bay‘ mā lam yuqbad" (selling what is not fully possessed) is a type of prohibited sale. The manner of "qabd" (taking full possession) varies from one commodity to another. If the commodity is real-estate, such as land, then taking full possession of it takes place when the seller releases the land and gives the buyer access to it so that he may dispose of it without any impediments. If the commodity is something movable, then taking full possession of it is achieved by receiving it by the hand, moving it from one place to another, or transferring it.