The World Federation One Stop Fiqh

Ruling 1778

If a person purchases something by undertaking to pay for the item later, and after the transaction he pays the seller money on which khums has not been paid,6 the transaction is valid but he will be indebted to those entitled (mustaḥiqqūn) to receive khums for the khums on the money he paid the seller.

6 This is referring to a type of purchase known as a ‘non-specified undertaking’. See the first footnote pertaining to Ruling 807.