Our poolbuilder just installed charcoal Marquis plaster rather than the pewter finish specified. We had picked the pewter finish because, when wet, it exactly matched the tile we had picked out - we were going for a monolithic look.

The poolbuilder is telling us it will be no problem to refinish, but that we have to wait 30 days for the existing finish to cure. They would then sawcut below the tile and chip out to make a slope - but that the finish for the most part would be 1/2" deeper than the current finish. So the walls will not be straight, and frankly I don't believe that a replaster would be of the same quality, not screw up the tile, etc. He wants us to pay for the complete job other than the 10% plaster payment and then wait for him to replaster. Technically, though, he's in breach of the contract - and my damages would be the cost of getting someone else to replaster plus the diminution in value of the pool for having been replastered. I have no idea whether that would add up to more or less than the 10% plaster payment. I think I should hold at least that much back. Any ideas how I could get a rough estimate of what that would be?