Client require budget to be commitment based on purchasing group.
Is there a way to include Purchasing Group in the source field of derivation rules in FMDERIVE?
Yes, you can do it as PO/PR number is available in COBL structure. Hence, you can read it and extract Purchasing Group using user-exit SAPLFMDT. Please, search the forum for more details on how to add fields to FMDERIVE.
P.S. Equally, check BADI_FMDERIVE (method FILL_CUSTOMER_FIELDS).