The art. 7th of the 57/68 Act establishes that the rights that the law establishes for the buyers cannot be WAVED. In addition, the artícle 1st states that the developer must give to the buyer a bank guarantee which covers principal plus LEGAL INTEREST.
Therefore, the right to get the legal interest paid is ruled by the law, so it cannot be waved. This means that any relinquishment document is against the law and void.
Should you have further questions, please do not hesitate to contact me.