i've actually just been working on something to do with tax exemption... so ur in luck (sorta!!) :p
the Qatari legal system provides for tax exemptions with the aim of encouraging foreign investments in Qatar... so in your employment contract.. i suggest that it includes a provision which stipulates the inclusion of tax exemption and cut downs provided for in Qatari Laws..
i studied in the Uk and covered EU laws but i havent covered taxation law so im not very sure... but i know for a fact that some countries such as France and Italy acknowledge these exemptions and deduct the Qatari tax exemptions from tax due from their tax payers..
so what u need to check is that wthr Spain's legal system is based on the principal of territoriality of tax (which im sure it is - but i have no solid documents infront of me to prove it so im not making a clear statement about it)

"He who reigns within himself and rules passions, desires, and fears is more than a king."