Hi @kristianharianja.  Thank you for your question.  Approval maps do not have the functionality to be triggered only based on the over budget warning because it is not a field stored in the database.

@kristianharianja although out of the box functionality can not work the way you explained as pointed out by @Heidi Dempsey but what you can do is to customize your PO with your desired business logic to retain required data from associated project budget line then utilize the stored custom data in PO approval map.