No, I've put up posters around here and in nearby veterinarians and pet stores, but so far, no one has called me.
Today I tried to contact the SEVEMUR (municipal emergency veterinary service) that is responsible for picking up injured or lost animals in public areas, and it was a complete nightmare. First, I called the city hall, they told me to call the municipal police, who then told me they would pass a note to the SEVEMUR to contact me. After hours of waiting, I called again and they told me that there was no incident registered. I got angry and went down to the street to find a National Police patrol and asked them to notify the municipal police because they weren't paying attention to me, and it's THEIR DUTY. The National Police officers were helpful and notified the municipal police, who finally contacted the SEVEMUR and got in touch with me. And here's the worst part: the girl told me they ONLY have 1 ambulance and 2 people on duty for the WHOLE city (we're talking about Madrid, the CAPITAL of Spain) and moreover, they barely have resources (cages, equipment, etc.). It's an example of the infamous cynicism of the Spanish government, who boast of being progressive and defenders of animal rights, and look at the conditions in which such a necessary service finds itself.
In the end, I've agreed with them that on Monday, I'll take the cat to them myself. I hope that before that, the owner or a foster family appears, because if they take him to the municipal pound, he'll have a bad ending there.
Also, I'm getting attached to the damn cat, and the longer it takes to fix this, the harder it'll be for me to part with him. Keeping him is totally impossible, as my dog killed his previous owner's cat (that's why he's my dog now, lol), and obviously, he's not compatible with cats.