spring a/the trap
To initiate, instigate, or execute a plan to secretly or deceptively take advantage of someone or something. The undercover agent waited until money had been exchanged before springing the trap.
spring a ˈtrap
1 make a trap for catching animals close suddenly
2 try to trick somebody into doing or saying something; succeed in this: The burglars were arrested after the police sprung a trap on them last night.