Unfortunately, the U.S. Embassy in Madrid does not offer tax assistance.
After more investigation, it´s a bit complicated but I think I´ve figured out all the paperwork I need to present. I need a certificate of coverage from SS here in Spain, which would prove that I am exempt from paying SS tax on the same income in the U.S. Unfortunately, the SS office here doesn´t know what I´m talking about. So hopefully I can find out exactly what form I need to be able to explain it to our dear friends the helpful civil servants

The U.S. deadline isn´t until April 15th, so if I can get this together in time great, and if not, I´ll file for the 2 month extension.
Any more advice is very welcome!!
