Derivation not working for Custom fields

Hi,

Please help me to guide with your comments for below problem which we are facing now with Derivation tool

Purchase Budget check is required at the time of PO Releasing Level not at PO Creation and PO change for specific PO types.

for this we did the following

1. Added the fields po type and PO Release indicator in COBL Structure.

2. Through the Exit we populated the fields in FMDERIVE

3. Created the Derivation Rule by putting the condition based on PO type and PO release indicator.

And Transactions tried as below

1. Created the PO with dummy Commitment Item saved.

2. When Releasing the PO , FMDERIVE trace is deriving the values which are given in the Derivation based on condition.

but when saved the PO , derived values are not saved, old dummy commitment item only showing.

we have checked in debug mode also COBL Structure is showing the New Derived values.

but that values are not saved

Can anybody give some comments on this.

Thanks on Advance

SAM