In theory. First of all you have to pay the amount demanded & I have never come accross anybody Spanish or other who has manage to get anything back from the Hacienda. Secondly who is going to pay for the legal & the opprtunity cost of the amount.
no you do not have to pay it first.
The only cost would be to pay for the valuations.
If you have someone act on your behalf, I assume they would not do it for free.
Are you sure about that? I had a client that told me that they had to pay it in advance. Never dealt with it by myself but only heard complaints about it.
I can only go by the situation I know of.
Buyer was told he had to pay an additional (approx.) 3.000€ of tax. he was able to argue it by using the valuation he had done on the property, which showed he had was buying for only slightly less than his official valuation. A new valuation had to then be done. Based on this last valuation he only landed up paying an additional 500€ (approx) in tax.
Of course the problem in Spain is that hacienda can take the money straight from you account, so you have to be prepared…