@Lisa PD
Good luck with your house purchase!
I would personally be a bit miffed if the developer didn't really lower the price, that's not much of a negotiation! :-)
But perhaps he has actually given you a discount? As most new property is sold as "Bulgarian standard" (i.e. unfinished), and you say that he will be doing both "finishing works" and "furnishing". If he's actually doing the finishing works and the furnishing then that represents quite a discount. If he's not doing any finishing or furnishing, then he's (a) not given you any discount, and (b) he's simply relabelling the costs in order to reduce the effective selling price, and pay less VAT.
It is, of course, officially illegal to under-declare a property transaction in this way. But it's very common, and often hard to avoid, even with larger developers. And, in practice, I don't think there's much risk associated with doing it. The developer is somewhat overstating it as "perfectly legal", but I'd accept it if he said "totally commonplace".
Separately, I don't actually know the VAT rules on property sales, but I purchased a brand new apartment and, as far as I recall, there was no mention of VAT. So I'm curious about which situations require it. Although perhaps it's because you're British (non-EU) and you're buying a house, and hence it's a side-effect of using a company for the purchase.
What's the impact? If you pay (I'll use some nice round numbers) 200,000 euros for a house, then VAT is 20% (usual rate, maybe there's a reduced rate for new property) or 40,000. That's 240,000 euros. As the buyer, you'll also pay the notary costs and the transfer tax, which are effectively another percentage of the purchase price: approximating at 10% would be another 20,000 (or maybe 24,000). That's 260k ish cost to you, with 200k ending up in the developer's greedy mits.
On the other hand, if you give the developer 50,000 in cash (black money), and agree to 50,000 of finishing and furnishing, then your purchase price is now 100,000 euros. With 20,000 of VAT, and 10,000 (or 12,000) of notary/transaction costs. Now it's 120,000 + 10,000 + 50k + 50k cost to you (230k, saving 30k ish), and the developer still gets 200k.
The developer gets the same money (so no discount given) and probably saves on his income tax on the profit too. 50k costs on 200k sale = 15k tax (10% of gain). 50k cost on 100k sale = 5k tax (10% of gain).
You also gain each year, as your property tax is based on 100k (the official sale price) rather than 200k (the actual price).
The downside comes when you sell the property in a few years. You bought for 200k, so you hope to sell for 250k, a 50K gain, and 5k income tax (10% of gain). But if you under-declare now, the official purchase price is 100k. So your gain would be 150k, or 15k income tax.