Hi again! Soooo. First of all try this website:
https://de.jobspotting.***/en/jobs/dortmund/business-administration
...and then I would suggest you contact Aiden. They are a new group who is developing a software for finding and offering minijobs. For now they are still working on their official website, so they just have a Facebook page. Here it is:
https://***.facebook.***/aidencorp/?fref=ts&__mref=message_bubble
Try and contact them, I'm sure they will be able to help!
Good luck with your search :D