Secondments
Hi Pablo,
This is an issue that is much more common among ESRs on ITN projects. It has been covered here and here years ago.
There seems to be many aspects that the TUM is getting wrong. (i) From the point of view of the MSCA regulations but also (ii) from the point of view of German law/regulations.
Contact your Project Officer for details about how the TUM should administer your grant and forward the feedback to admin.
The mobility allowance is given to cover extra costs caused by your mobility to Germany, that is, costs of returning to see your family and so on. Their argument is completely bogus.
The TUM is also afraid that you will claim a long business trip (Dienstreise) for the whole secondment: This would cover accomodation, travel expenses and a per diem for your living expenses. Technically the most correct way under German law to reimburse your secondment is as a "Dienstreise".
I have certain understanding for ITN supervisors, if there is a convenenient way to swap accommodation between ESRs moving between hosts in opposite directions, I can see why they might want to put more money into other parts of the project. Unfortunately swapping accommodation is not always feasible and certainly not for an IF.
Regards,
Brian