Is there a way to create on screen validation on 'Enter Requisition Line' screen?

Summary

We want to create on screen validation on 'Enter Requisition Line' screen in order to restrict the 'Deliver To Location' options

Content

Hello guys,

We have a requirement from our customer that they must prevent a purchase requisition from being created if the requester creates a requisition with an item derived from an agreement and which has a different delivery location than the one specified in the agreement.

In order to do that, we created a web application on VBCS where the user can enter the agreement number, business unit and allowed branch offices (Deliver To Location) for that agreement and saves it on a Business Object. The problem here is how to create the validation on Purchase Requisition screen considering the data on that BO.