The Consumers' Office
Every municipality in Spain has a public office dedicated to informing you, the consumer, regarding your rights and responsibilities in the case you should have any doubts. The offices are known as "OMIC", which stands for "Oficina Municipal de Información al Consumidor". Ask at the town hall to find out where yours is located.
Among the questions these offices deal with are those having to do with contracts, guarantees and transactions between shop owners and clients. It is best to bring all receipts and paperwork with you when you approach this office as the advisor assigned to you will usually be happy to go over everything with you. He or she will also need a paper trail in order to point you in the direction of justice should that be necessary.
Many, many complaints reaching these offices involved informal, verbal contracts. For this reason, experts advise always putting everything in writing. Each and every change to the original agreement should also be put in writing.