What jumps to 120 volts, the white wire to neutral buss bar?
Are you using a digital volt meter or a solenoid type?
Are you disconnecting the white wire with the circuit hot?
You sure it's the correct white?
Where exactly do you get 120 volts, from where to where?

Is your wiring capable of carrying 50 amps you should have #6 wire to device. Why are you taking white wire off, if the breaker is off, disconnect load side, remove breaker, install new breaker, connect load wires. 220 is measured across the two hot leads. when you reference either side to neutral or ground you wuill have 120 volts. Now having said that is your neutral floated and not connected too ground side. The problem of floating a neutral wire is that if you touch neutral and ground you will receive shock.. Best to call an electrician.