Hi,
If you have two-way match :- why you are going Evaluated Receipt Settlement(ERS) procedure?
Evaluated Receipt Settlement(ERS) is the procedure for settling the Goods receipts automatically where ERS allows settlement of goods receipt without receiving actual invoice from vendors.
Based on price maintained in purchase order and quantity captured during goods receipt- system determines the invoice amount,generate the corresponding invoices,post them and hence a vendor invoice is no longer required.
If somehow you are posting invoice - you can set tolerance limits for tolerance keys (t.code:OMR6), so that if you are doing invoice more than PO amount, system will restrict you by showing error message.
Regards,
Biju K