Amount of cost out of control forBlanket PO

Hello, I met one issues as blow,

1. Created a blanket PO in the ECC6.0 EHP8 system and the total cots is 3000 euro

2. Two invoice I received, one is 2500 euro, another is 1500 euro

3. Use T-code: Miro to deal with two Invoice,but not post finally , only hold it, it means that two invoice be holed

4. based on 3 rd point, continue to post the invoice which be holed hold , and pots successfully.

4. check the PO, found in the items, the actual is over overall limit

could you please give me the suggestion how to avoid this issues happened?