Zero Out Commitment

We have found some instances where bills were applied to commitments so the commitment is currently overstated.  I need to reduce the amount of the commitment by $200,000. I can’t edit the commitment itself and for some reason the change order screen is not allowing me to do a deduct change order of -200,000 (it’s not liking that it’s negative for some reason).  How am I to reduce the amount of a commitment??