One of the preconditions for a valid sale under Islamic law is the existence of subject matter at the time of sale. However, there are two exceptions that allow the sale of nonexistent objects. The exceptional sale contracts in Shariah are Salam (forward sale) and Istisna' (manufacturing contract). Salam is a sale whereby the seller undertakes to supply some specified goods to the buyer at a future date in exchange of an advanced price fully paid at the spot.