It used to be not just very common, but expected, that property transactions were under-declared in order to avoid transfer taxes and capital gains taxes etc., and everyone involved in the process, estate agents, lawyers, bankers and even notaries accepted this as normal. Things have changed somewhat, as the government has cracked down on this tradition, and anti-money laundering regulations have become tougher. Now, if you under-declare the sale price, you run the risk of the tax department presenting you with a "revised" figure on which you have to pay tax, and possibly a fine to boot.
In the current market, I'm not too sure how anyone can determine what the actual price was in any transaction, because without doubt there are some distressed properties genuinely changing hands at hugely discounted prices. So I'm not sure how reliable this is now, but this link provides a calculator to give you the minimum value to declare. You need to enter the valor catastral (rateable value) of the property, which can be found on the IBI (rates) receipt, the municipality where the property is located, and the current year. Out of interest, I just entered my own details, and the minimum suggested value for my property is about 40% less than what I would hope it is worth, (even today, which is probably about the same as I paid for it in 2004), and probably not coincidentally, pretty close to what I declared when I bought it. Personally, I would not risk under-declaring by so much now.
As long as the tax department is satisfied that you have declared a realistic figure for the sale, I see no reason why you cannot agree with the seller, by private contract, a separate figure for "fixtures & fittings". The seller, particularly if they are Spanish, may readily agree to simply receive part of the money "black".
I strongly suggest that you use the services of an independent lawyer to advise and guide you through the whole process.