Is paying a deposit to reserve Islamic tapes that are currently unavailable considered selling what one does not possess?

If the specifications and quality of the tapes and the material recorded on them are precisely defined, and the full price is paid at the contracting session, then it is permissible to sell them before possessing them, because this falls under the category of Salam (forward sale). However, if their specifications are not precisely defined in a way that removes ignorance (Jahalah), or if the price is not paid at the contracting session, then it is not permissible to sell them; because in that case, it would be selling what the seller does not possess, which is prohibited by Islamic law.