Hi Ammar,
Through this article (****://***.hna.de/kassel/fragen-antworten-thema-wohnungen-fluechtlinge-6100728.html) I found out the contact person for Caritas in Kassel. She apparently helps to coordinate available apartments with refugees.
Her name is Janina Haupt and her contact information is:
janina.haupt@caritas-kassel.de
0561 / 816 427 -10
Here are three more contacts through Landkreis Kassel who also should be able to help you find something:
Frau Wilburg Kleff
Tel: 0561 1003-1841
E-Mail: wilburg-kleff@landkreiskassel.de
Frau Susanne Baberuxki
Tel: 0561 1003-1534
E-Mail: susanne-baberuxki@landkreiskassel.de
Herr Karsten Nickel
Tel.: 0561 1003-1846
E-Mail: karsten-nickel@landkreiskassel.de
Good luck!! Let me know if you have any other questions :)
Julia