By we all know how Upen Patel uses girls for his benefit in the show, this has been his only working strategy in the show! Even outside the BB house he has got a Casanova image & broke many women's hearts cheated on many (do a little google on him & you will be shocked)! I think he is playing with Karishma's emotions ust for the sake of money whereas this girl has genuinely fallen for him.
I think as par news Upen is dating Natasha yet when he came to BB house during finale dance act he proposed to Tanna ( Dimpy is definitely not bluffing) . He did that knowing it very well that BB team will notice the act. The house is completely wired with cameras & they wont miss him proposing her. He ust faked the proposal but Karishma took it seriously as she is our very own GK-Queen!!! Even Dimpy did not understand his game.
He did all that ust to put up a show in front of BB creative team. Then he approached them for re-entry & they gladly agreed expecting lot of romance leela!
Upen only cared for money & definitely not Karishma. He will go out with some money, have fun with Suku/ Soni/ Natasha & go back to London. Whereas Karishma will loose the game, loose her BF & will be evicted with a broken heart!
I think this is what the actual story is, in contrary to what most of the people are thinking.