Amending of Complex PO

Dear Gurus,

We have implemented Purchasing and Service Procurement, for Milestone related purchases.

After the approval of Complex Purchase Order increase in the PO value can be done by adding another line to the existing approved PO.

But if we need to reduce the PO value for the Complex PO system is not allowing since PO Amount value is freezed and not like other PO types.

This case we can't cancel the PO already partially Work Confirmation and Payments made against the PO.

What is the solution for this?

Regards

Ram

Note: The complex PO's are getting created against Project and Task.