The commitment values are saved in the database table COOI. For purchasing orders, the commitments can be value based or quantity base handled, based on the unit of measure used on the PO.
For positions that are using quantity-based commitments, the commitments will be reduced based on the quantity from the goods receipt/invoice and not in the value.
In customizing, the unit of measure can be checked via transaction CUNI [since 40A], or in the field T006-KZWOB (set means value related).
(Customizing -> General settings -> Check unit of measurement).
For purchasing requisitions, the commitments reduction can only be valued based for service items. For non-service items, the purchasing requisitions commitments reductions is always quantity based (note 355793).
But in order to reduce the balance commitment, I would recommend you to create a background program that will collect the purchase requisitions that have valid PO created and put a close indicator. I would recommend to set up a time frame so that program take very old PRs into considerations and even newly created PRs.
You can also explore use of ML91 to activate commitments for services at services line items.
Also Refer some additional notes below:-
355793 - Note:qty-based reductn of purchase requistn commtmt
639523 - Commitments reduction behavior for Purchasing
534993 - Short instructions RKANBU01
152571 - Composite Note: Missing or Incorrect Commitments- CO side
670489 - Commitments for delivery costs