PO Change

PO Prepared with conditions

i.e. Net Price Rs.100/-

Plus Excise 10/-

Ex.cess 2/-

Hi. Ex. cess 1/-

-

Total Amt Rs.113/-

Advance Paid Rs.10/-

Now, excise rate change @8/-

I want to change the excise in PO , sys not allowing to change as advance already paid.

Advance can not be reverse as already sent to vendor.

Pl help me how to sort out the issue.

Thanks & regards,