Hi,
You are creating a contract and from with reference to contract- you are again creating a PO and you want change PO price in ME22N without altering contract price in ME32.
You can play with condition type and change PO price as & when required by you.
Use t.code: M/06, SELECT check boxes of "Value" and "Amount/percent" in 'Changes which can be made' segment for the condition type PB00 and PBXX. Also correspondingly update the change for condition types PB00 and PBXX in your Pricing procedure in t.code:M/08.
Create a new PO from contract and now try to change your PO price in t.code:ME22N / ME22 which is created from contract and see the difference!
Regards,
Biju K