MSCA GF from Spain, Madrid, social contributions

Hi,

I will soon start the outgoing phase of my MSCA GF. The beneficiary is the University of Madrid and in the outgoing phase I will be in New Zealand. When I got my working contract, I realized that the payment is way lower than I expected. I figured out (and many before me I thinkg ^^) that this is since the UoO deduced all employer social contributions from the grant money before calculating my gross salary. This deduction is in Spain about 30%. That means, my gross salary is about 57k per year instead of 74k per year. Still okay, but a bit disappointing since it might give me less money than a standard NZ post doc contract, depending on how this will be taxed (in NZ or in Spain).

I have a few questions, maybe someone was in a similar position and/or knows a bit about Spanish working laws:

- is there a way to opt out of some of the social contributions either on employer or employee side? Obviously I will not go to a doctor in Spain and I am not sure whether I can even use my pension if I go back to Germany (where I am originially from).

- how do I handle this tax wise? Do I pay taxes in NZ or in Spain, or in both countries? (they have a double taxation agreement, but I dont fully understand how that works)

thank you very much,

Florian