Those Germans know how to make quality tools.
Apparently they did even in Paleolithic times haha
Lol.
The father in law would have known who made the obsidian knife. Perhaps if his Mom is still around, she might know.
He mentioned estate sale, so it sounds like he passed or bought it at an auction.
The stamp "Made in Germany" on the empty sheath is all I can see of origin of a totally different steel blade that isn't in the picture, but probably quality. The reinforcement rivets added to the stitching isn't very common in my limited experience, but assume it's to protect stitches when resheathing the blade. The snake skin and cord suggests a handmade sheath.
IMHO, the value will be much higher with positive ID. Perhaps a knife makers forum would shed some light. I would also check all obsidian knife makers in that area and call around.
PS: DBGM is stamped too. That might just get you somewhere if there's a blade to go with it.