While I do not support any form of tax evasion, I would be a little less hasty to cast the first stone at this builder without knowing more.
In the current climate, when a new house sale closes, the bank who hold a mortgage will want all of the nett sale proceeds. They will want to apply this against their loan. They will not be willing to release any of this money to their customer to fund his continued work, to reduce the overdraft he has run up in finishing the house or indeed to fund his VAT liability arising on the sale.
Even good, asset-rich builders are in a situation where they are finding it difficult to get money to finish houses so that they can complete already-contracted sales. Acting for house builders now means negotiating with the bank throughout the construction period and beyond.
It may be that this builder is trying to circumvent unreasonable limits imposed by his bank. It may even be that the €25k he is looking for 'under the table' is in fact required so that he can discharge the VAT on his sale, and will be declared to Revenue (but not to the bank). He would be better off getting his solicitor to negotiate proper terms with the bank ( as my clients do..........) but perhaps he does not have a solicitor or accountant prepared to go to bat for him. Perhaps he is one of those builders who have left themselves without good advice because they have not prioritised their relationships with their professional advisers.
Of course, the most likely explanation is that the builder is going to fiddle his taxes. But these are strange times - so as stated, I would not leap to judgment.