CPA currency in BPM rules

We are implementing a BPM rule which would verify the currency of the CPA and the currency which has been entered on the requisition, once the user selects that particular CPA, if there is a mismatch in the currencies then the requisition should be rejected during approval. For Ex- CPA currency is USD and on the requisition, post selecting the CPA, the currency is GBP, so GBP ≠ USD which should reject the requisition during approval. Currently we are unable to find any attribute for CPA currency. Please advise if there is any.