Why do vendor credits not reduce available room left on amount of commitment?

SOLVED

When a vendor sends their credit note related to a commitment and subsequently provides their corrected invoice bringing amount invoiced to full value of commitment, we can't successfully enter the subsequent invoice and have to create a POCO to increase the commitment amount. Only then will the final invoice be able to be processed but that seems counter-intuitive.

Are we doing something wrong? Is there another way to accomplish the goal of having invoices less credits equal the committed value?