When I bought my property here, I saw it in the agent's window, but instead of going in, I took down the details of the property, went home and searched for it on the internet. I saw it advertised on a website and called the owners directly. We haggled on price, I got them down a few grand and bought it without going through the agent. There were no problems with the purchase and I saved thousands.
I don't know if the law is the same as it was seven years ago but if you made an enquiry for the property with the agent and then decided to buy it, you had to do so through the agent. If not, they could make a claim against you. However, if you simply went directly to the seller, you were under no obligation to the agent, and as the commision is charged to the buyer and not the seller, the seller wasn't obliged to use the agent either.
Of course, if you really need peace of mind, the agent maybe the only way to go, but in Spain it's very common to buy and sell property without using an agent.
Kelju,
Have a search on the internet to see if the property is still for sale. You could still be in luck.