Hi Detlef,
While the origin of the knife itself is still somewhat mysterious, I'm pretty sure the sheath is African.
In addition to the general look of it, the belt loops and their fastening are typically African (for example :
https://www.quaibranly.fr/fr/explore...-gaine/page/6/) as well as the stitching and type of leather used.
I might add that I picked this knife from a lot of North-West African, see picture attached.