Mendy recalled on Telefoot: "I told him: 'if you score a double, I will buy you a watch', he said to me: 'go ahead, you will see'.
"He then scored two goals!
"At the end of the match, I talked to him about the result: 'It's okay, we're fine, we have the advantage'. (But) he said, 'bring the watch!'."
Mbappe and PSG go to BVB this week for the first-leg of their Champions League round of 16 tie.