Is there a way to validate the data from a Business Object on Purchase Requisition creation screen?

Summary

We have a requirement from our customer in which we need to validate some information in the Purchase Requisition creation screen

Content

Hello guys!

We have a requirement from our customer in which it's necessary to check whether an item listed in a purchasing agreement can be requested in a purchase requisition based on a branch office location. If the requester selects a location in the purchase requisition that's not listed in the purchase agreement, an error message should appear or the requisition shouldn't be processed.

We did some tests and found that it's not possible to select more than one branch per business unit in the purchase agreement creation screen and we created a custom screen on VBCS that stores the branch locations and some other information on a Business Object.