For the first scenario and will take this scenario to this service request.

1. Create a PR and added line.
2. That item type is service or procurement category, so as per your business requirement quantity and should be blank and Net amount will be entered.

Second scenario:

1. Create Purchase agreement.
2. Added line for (service item/procurement category) and there is standard behavior is quantity field is not editable when we select value/product category value commitment. which works fine, at the time of conversion to PO those lines entered in purchase agreement has to come and ask for quantity and value.
Ideas Administrator

Dynamics 365 has a fundamental requirement of having quantities on the line when posting.