Authorization Rule Based on Department

I want the below task to be performed.

  1. A User who is the approval authority for the Purchase Order of two departments.
  2. One department he can approve PO of grand total of Rs. 10,000 whereas for another dept. he can of Rs. 15,000. If more than these values it exceeds, he can’t submit PO. It will be done by Head.
  3. In Authorization Rule, it is not possible to do because wither I have to select a user or role but then it will be applicable to POs of all departments.