Custom invoice blocking reason

Question

When posting an invoice, several checks are done to see whether invoice items should be blocked for payment. This is being done based on price and quantity. Right now we want to block payment for an incoming invoice item if that item does not have a corresponding PO item.
To this end we want to (ab)use the blocking reason for quality. When posting an invoice, there will be a time in the code that blocking reasons will be checked, at least for price and quantity. At that point we would like to insert our own code to set the quality block field if required.
Is there a way of customizing this process or creating an additional check without having to adjust standard SAP? Thanks in advance for the help!

Answer 1

An update and a question on the issue. The badi eventually didn't work for us, the additional checks it performed made it very hard to work with. What we did was adjust standard SAP in the routine for quality check. If our check returned negative, we set the quality block and return the process to SAP. Luckily this isn't a coding beauty contest, but it works perfectly.
Now we've been asked to implement the same process in another system. Newer system (SAP ECC 6.0), so once again we want to check whether there is a better way of solving the issue.
We know we're able to define custom blocking reasons. However they all seem to be related to tolerance level checking, which is not what we want to do.
Is there any way of creating a custom blocking reason on header or item level, which we can set with custom ABAP coding? Then we can program what we want to check and when to set the block.