I think the entrepreneur has responded here to his adjustment of the contract offer in a really nice and friendly manner. This is rather the exception than the rule. And the original poster (OP) also saw the limitation directly, so it was not hidden. Here, the two parties did not agree. The seller possibly assumed something that the entrepreneur does not want / cannot provide. Presumably, this had never been an issue before, so the seller’s statement may have been somewhat premature, but basically not malicious.
Why should the entrepreneur then be forced to stick to this wording and consequently not realize the calculated profit from the construction project? He needs the margin that results from his calculation. He probably does not run the business as a hobby.
Since he knows that sellers are allowed to make some concessions, he reserves a right of correction in case of overly dynamic results. He also writes this directly into the contract. Usually, it is indeed accepted that way. The OP - also aware of this formality - probably assumed this as well. Now, however, the situation is different – which is explainable. Why should he give his sellers such a far-reaching power of attorney when he plainly knows about such circumstances? I wouldn’t do that either and would not think badly of it.
Everyone may come to "their" own conclusion. I cannot understand the many negative insinuations here. Especially not that they escalate into insulting remarks. These may sometimes be appropriate, but in my opinion absolutely not here.
Make peace.
No contract was concluded here, and that’s that. This was recognizable and, in the end, absolutely correct. Personal disappointments should be dealt with in a different way.