During a discussion about your implementation, your client mentioned that they negotiate credits for equipment...

A document style is a template that defines the layout and content of a purchasing document, such as a purchase order, a purchase agreement, or a requisition. You can create a document style that supports credit lines, which are purchase order lines with a negative amount to record credits to the ordered amount that you have negotiated with your supplier. For example, you can use a credit line to represent credit for an equipment trade-in. You can also specify other attributes for the document style, such as the line types, the change order policies, the approval rules, and the communication methods.

References:

How You Create Purchase Order Lines with Negative Amounts

How You Enable Creation of Purchase Order Lines with Negative Amounts

Create Supplier Contracts from Negotiation Award