bargaining chip 
(British also bargaining counter)
noun
A potential concession or other factor which can be used to advantage in negotiations.The territory could be used as bargaining chips in any negotiations....
- However, rather than engage in hasty strike action SIPTU is expected to use the threat of disruption as a bargaining chip in forthcoming restructuring negotiations.
- But a decision to sell off its shopping centre interest would be a major surprise as it gives the council a huge bargaining chip in negotiations over town-centre regeneration.