Bargaining chip

Bargaining chip - something that can be used to gain an advantage when you are trying to make a deal or an agreement.

Example - "The workers used the threat of a strike as a bargaining chip in their negotiations"

Image - http://img1.cdn.tradew.com/Y201308M1552805C1754754/T6G4060321/W499H499/HF+RFID+Bargaining+chip+C-1041.jpg