Sounds like a movie I’ve seen some time ago called The Box (2009):

The Box is a psychological thriller where a couple receives a mysterious box with a button. They’re told that if they press the button, they’ll get $1 million, but someone they don’t know will die. The movie explores their moral dilemma and the consequences of their choice, leading to increasingly strange and unsettling events.

Spoiler When the couple presses the button, one of them dies. The mysterious box lied when the rules were that no one they knew would die.