Why not back up your claim?

After a medical procedure, my medical aid left me with an amount of R2 739 balance. Three times I wrote to our GAP cover and three times they responded "the medical aid did not settle the full amount as the healthcare provider has an agreement with your medical aid and you are not liable for the shortfall" Twice my docter denied that he has such an agreement. I cannot get Zestlife to answer, or supply me with a proof of my docter's "agreement".