Buying property abroad is not a venture to be undertaken lightly! It is not wise to assume that the entire buying process will follow the rules we are used to "back home" and generally speaking, there is often a new set of rules and regulations, traditions and pitfalls to learn, understand and beware of.
Spain is no exception and has a tangle of laws and legislation seemingly designed to trip up an unsuspecting real estate buyer.
So what should someone hoping to buy property in Spain do if faced with such a situation? Should they agree to the vendor's demands and hand over a brown paper bag filled with 500-euro bills? For a more better solution one can also refer to sites like https://www.homesgofast.com/property/sale/spain for buying property safely in Spain.
If they do, then the price for which they appear to purchase the property will be the price against which future profits from resale of the property will be compared for capital gains tax purposes.
Therefore, if the buyer ever wants to resell the property, he too must insist that his buyer agree to a reduced sum in the contract as well; otherwise, he will be responsible for all additional capital gains taxes! Someone somewhere along the line is going to lose, just make sure it's not you!
Think hard about what you are being asked to do and the possible long-term legal and financial consequences rather than your short-term excitement about securing your dream home in Spain.
The next point that deserves due attention is the fact that there are many unregistered and inexperienced real estate agents operating in Spain.
When looking for a real estate agent to represent your interests, find one with a strong, positive mindset that will go out of their way to protect you, and therefore is unlikely to want to offer you anything less than first-class service.