Hi Tourist -
I cannot for the life of me understand why you say “obviously they are regretting it”. They absolutely should be cheering about it in my opinion!!!
Seems to me they have done exactly what most of us try to do, and that is to minimise the “escritura value” of the propert at the time of purchase - to save both the buyers and sellers paying any more tax than necessary.
If I recall Purchase tax is about 5% of declared escritura value, so they saved themselves a bucketfull of tax by paying a lot of the real price in black money. No doubt the seller also saved a load of capital gains tax (used to be 35% on the capital gain for non-residents, now equalised to 18% for everyone if I recall) by accepting a load of black money.
Now it’s time to sell, so just do exactly the same; as everyone else does.
Talk to a Spanish lawyer or Gestor. They’ll tell you by roughly how much you need to increase the “escritura” value of the property in the intervening years when you come to sell, in order to stay within the normal limits of annual increase which the Hacienda uses so you stay below their radar.
Advertise the property for the real value that you want (that won’t be what you get of course, as here in Spain everyone asks lunatic money for a while untile they finally realise that those days are over, and if they really want to actually sell the house they are going to have to drop the price a lot. I mean a lot.)
Then, when prospective buyers make an offer, tell them that you’ll accept so long as the escritura value to be declared at the Notary is no more then “X”. Rest of purchase price to be paid in black money at the Notary’s office…...as happens twenty times a day on every day of the week at every Notary’s office throughout Spain.
Result? You save a bucketload of capital gains tax, and buyer saves a bucketload of purchase tax. Everyone happy. Again.
Just keep the black money as cash, or feed it into various of your bank accounts in small injections over the following year or two to stay below the money laundering limit (used to be 6000 euros per deposit, or 5000 GBP’s I think).