I am not a lawyer, I am not a realtor. But I do believe that contracts can be changed if BOTH parties agree to it. That is a lot of what happens during the whole negotiation process. You make an offer ( contract) they modify it and make another offer (contract). The contracts ( called contingency agreements) can change anytime up until closing.
It sounds more like the realtor wants the extra commission. When you began the whole selling process the realtor could ahve agreed to take 4 or 5% instead of 6. It is a choice. I am willing to bet they still just want to keep the extra commission. But on the other hand, the realtor would have to eat the whole 1%. It is unlikely (although possible) that BOTH real estate agents would take a 1/2% off their commission. That is what it would take. Agreement by BOTH realtors. Plus it might be necessary to involve the agencies involved as they get their cut also.
There is a much more painful option though, wait a year or so. See prices drop even more. You will lose the $5k and more.