Definition of Emptio rei speratae
A purchase in the hope of an uncertain future profit; the purchase of a thing not yet in existence or not yet in the possession of the seller, as, the cast of a net or a crop to be grown, and the price of which is to depend on the actual gain. On the other hand, if the price is fixed and not subject to fluctuation, but is to be paid whether the gain be greater or less, it is called emptio spei.
That's the definition of Emptio rei speratae in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.