Was the song created with Magix or is it a .wav or .mp3?
If it's a .wav or .mp3 then import it as usual. Magix will default to 120 BPM because it doesn't know the tempo. When you change the BPM to 100 it assumes the song was 120 and slows it down.
To correct this simply right click on the imported song and choose the remix agent. It will walk you through the process of establishing the correct BPM for your song. Once done you will be at the correct tempo.