that is a different kettle of fish altogether. Be aware that this is a practice that some estate agents use to mask their commissions to the buyer and seller. NEVER sign an agreement between agent yourself. It should always be between you and the seller directly because if anythign happens to the buyer there oculd be problems later on.
I know of exactly this incident happening where the agent signed between the agent and seller and agent and buyer – there was a 11 month timeline because the seller weas having a hosue built. In the meantime the said agent died, the place closed down and the buyer had no recourse because she signed between her and the agent – ever tried getting yourmoney form a dead man?
So be VERY careful signing with the agent – INSIST on a direct contract between you and the seller. Also insist they give you a liquidacion de venta which breaks down the costs of sale – including commission and IVA – it is your right, this should be signed by both you and the seller. You may be asked to take this to the notary in future as a new law coming in means the Notary has to see sight of all contracts.