回複：Please advice, how to ask for promotion.

Sounds a weird situation if not a trap, or they are 忽悠you. As far as I know your rate is set by B and they got different amount of money from A. If they want to pay you more, they will get less unless they want to renegotiate with A. So you see where the possibility can be. Besides, if B didn't use the word "raise", promotion doesn't mean anything.