Property purchase Tax

If you pay less than the cadastral value then yes, they can later come back and demand the tax on the difference. 

It's a systen to avoid the practice of deliberately undrevaluing and paying a seller a wodge under the table, you said you'd "negotiated a good deal" and unfortunately this is the potential downside of that. 

Sounds to me like your solicitor is playing it by the book though and trying to avoid you getting a bill sometimes years later, the €500 is probably for a new valuation but it's a risk because if it comes back higher then it will cost you even more!

At least your abogado has warned you about this in advance as many people get a nasty surprise a few years down the line. Other posters have had this done for less than €500 so it may pay to shop around, possibly find a gestor instead as although there is no guarantee, values have been successfully challenged.
To have a Complementary Tax bill of €4k, that means you are saving €36,000 instead of €40,000 which is a different way of looking at it but it is hard to put sense to it without knowing the price of the property. If you are paying €100k, a difference of €40k is going to take more arguing than if it's worth €500k.........

Hi RayD

I know exactly what it is as we have had properties in Spain before although these were new at the time, our first did have a bag of black money as they call it the second I refused to pay that way so I know exactly what they are trying to aleviate.

My situation is they are wanting me to pay tax on their valuation of 141k on a 2 bed apartment which I am paying 105k for all legit money going through banks and solicitors, these properties are up for sale for between 118k-130k gaining no more than 110-120 at the very best, its wrong what people have done me included but I had no choice as we had paid a deposit and told later of this practice.

When I say a good deal that is what I meen not that I have fiddle anyone, its the same old thing you are given a break down of costs you budget to and then hello oh no there is this and this to add and oh it will cost an extra €500 to sort this no other place does this thats my gripe and always has been with Spain.

Hope I haven't offended antone with this but it just puts a bad taste in your mouth when they do this.

Regards Andrew

Hi Aspen,

Here's a link to a previous post I've done on Complementary Tax - the information is from my tax advisors, Abaco:

https://www.costablancaforum.com/area/formentera-del-segura-spain-54/taxes-suma-nie-tax-advice-in-formentera-del-segura-25/complementary-tax-little-known-often-misunderstood-and-can-be-expensive-58217/

It's worth noting that Complementary Tax can be successfully appealed, if an incorrect calculation has been applied.  You could take the view that, now you know of the possibility of a bill, wait to see if you receive one and then appeal it, but note that you only have a month to appeal, once the bill has been issued.  Currently, the tax office only goes back 4 years, so if you don't receive a bill during that period, you're unlikely to get one.  On the other hand, you may feel that, given the amount of money you've saved on the purchase, it's better just to pay the tax now.

Best wishes,

Kim