System not checking Budget during Condition change in PO

Dear Friends,

whenever we change in conditions of the PO, system is not checking budget for the same.

I do have Account Assignment WBS provided in PO. with AVAC activated on same.

take an example

WBS Budget Rs.100.00

PR Created RS. 90.00 Acc.Asgnmt. WBS

PO Created RS. 98.00 WRT PR+Conditions

Now PO Conditions need to change

PO Change Rs.105.00 updated

Commitment value updated in COOI table, but system skipped Budget checking

Please let me know Why system behave like this??