We had a similar situation here in Valencia province and had to pay c8000 Euros eighteen months ago and within four years from our buying the property.
I think that they have up to four years from the purchase date to start the procedure to ask for more money. After discussion with various lawyers and advisers we were definitively told that we had to pay. If we do not pay then we were accruing further interest and eventually a debt would be added to the house so we bit the bullet and paid it. It makes selling a house with a debt on it very difficult
The eventual reason was that when we bought the house we had a small mortgage both in terms of amount and term. Therefore on the deeds was the bank valuation which was considerably more than the amount we paid but for tax purposes the authorities insist that the value was the banks and the tax had to be paid on that higher figure.
We do know off people in a similar situation indeed one case were the owners had to pay over 25000E.