Final Bill dispute with a Contractor (Plumber)

I have a rental home that needed pretty extensive plumbing work and the plumber I hired gave me a $6200 estimate. As the job progressed, more work was in needed and I agreed verbally to an additional $2000. When the job was finished, the plumber presented me with a final invoice of $10,380. I questioned him on it and he simply says that the materials and labor added up to more than he expected, but that his invoice is accurate and he wants full payment. He provided me with a breakdown of his and his employees labor and a material list, but I have no way to verify the accuracy of his labor and most of the materials are inside the walls and attic and I cannot confirm its accuracy.

I offered him $9000, but he wants the whole amount.

I think he is trying to pull one over on me, but I don’t want to risk a lien being placed in the house.

Should I continue to challenge him, continue to negotiate with him, or simply send him a check for the original $8200. Or, should I consult an attorney?

Thanks…