Replacing a 20A 250V outlet
I'm going to risk posting my comment as an answer.  If the original outlet was truly a 240 volt outlet (probably a cable of 2 wire and a ground) and you didn't change the wiring in the main panel by moving the white wire (of the circuit) to the neutral bussbar making it a 120 volt circuit, your still going to get 240 v on your new outlet.   The outlet has no way of changing 240 to 120, that's the role of the breaker panel connections. That assumes, Ed's comments about a MWBC dont' apply Hope this helps.