SES problem

Hi guys,

We have created a service PO with net rate 100-00. But vendor invoice we have received with 90-00. Can we change the rate to 90 at the time of service entry sheet. Our business requirement is that they do not amend the PO, user will change the rate as per invoice if it is less than PO rate. If it is more than PO rate, PO will be amended.

is it possible to do this? How to do this?