Take no notices of anyone requesting you to pay black money. If it means you lose your dream house they so be it walk away. Try to ensure that the vendor of the house you are going to buy knows that you propose to fully declare the value of the property, I have seen some good arguments even at the notary about this.
lots of people will tell you this process of underdecleration is normal in Spain.
Normal it may have been in the past but things are changing.
illegal it certainly is, not only will you guilty of Tax evasion, you will also be aiding and abeting the seller in tax evasion.