Istisnaa sale

Q: Can you work for a shop which shows their clients samples of blinds and thereafter they acquire it for them? I don't think the shop owners understand that it is not permissible for one to sell something which one does not possess.

Restraint of trade agreement

Q: I would like to know if a restraint of trade agreement is permissible in an Islamic sale.

Example: I am selling my cellphone business to Zaid. In the sale agreement a clause is added that I, the seller, is not allowed to trade in the same field of business for a stipulated period (e.g. 5 years). Will such a clause be permissible? If not will the adding of such a clause nullify the entire sale? Or will the clause be impermissible and the rest of the sale be correct?