If you are currently enrolled at a German university you should find out whether your university is taking part in the "Programm zur Steigerung der Mobilität von deutschen Studierenden (PROMOS)". A participation in the ISSEM can be rewarded with a PROMOS scholarship e.g. for specialised courses (Fachkurse) or language courses.
The Humboldt University is not taking part at this kind of PROMOS scholarships.
Please note that this application runs completely independently from your ISSEM application.
Scholarship via study foundations
Many study foundations offer scholarships for summer school programs. If you are in a study foundations, check which kind of scholarships they offer.