Hi,
You created Z3 tax code for PO and done Goods receipt playing with MRP indicator in MIGO screen!
Now you need to used suitable tax code( the excise value matching with which you entered during MIGO screen) which is different tax code in MIRO screen and complete invoice verification.
Refer a thread similar to your requirement.
http://scn.sap.com/thread/2077113
Regards,
Biju K