Kim Kardashian -- Must PAY Kris Humphries to Keep Engagement Ring

Kim Kardashian is going to shell out some serious dough so she can keep her MASSIVE engagement ring ... and TMZ has learned, the cash is all going to Kris Humphries.

Sources close to the former couple tell us ... it was Kris who originally bought the 20.5 carat diamond stunner ... and Kim loved the ring so much, she included a clause in the pre-nup which allows her to buy the ring from Kris for the original purchase price in the case of a divorce.

Ironically, without a special clause in the prenup, Kim would have been the proud owner outright, because once they became man and wife, the ring would have been Kim's.

Since the couple is now kaput -- the diamond is in Kim's court. We're told KK still LOVES the ring -- and will pay Kris so she can keep it.