it wouldn't have been pimperish otherwise. The whole notion of the film was that he had planned the perfect crime & that included evading capture even after several direct confrontations with the pursuer.

Plus, he bragged earlier in the film, he was just going to walk right out the door.

as everyone else up said, I don't think he was giving him the diamond just to rub it in his face. but I think that was a little part of it, but not the main part. And not so much to rub it in his face, more like to big-up his own skill.