I'd be skeptical of its provenance. There is a ton of replica Third Reich material out there, some of identified as such, some of it not. The odds that a cap belonging to an officer from the U-124 survived its owner are very long.
There are illustrations of U-Boot crew wearing their unit badges, which were unofficial, by the way, in Squadron's "U-Boats-In Action" book. If you have access to a copy, it's worth taking a look.
The Wehrmacht also allowed similar badges to be worn on caps, some of which were sanctioned, because the given units had inherited the tradition of old Imperial regiments (like the Totenkopf of the Leib-Husaren, the Totenkopf of the old Brunswick Infantry Regiment, and the Schwedt dragoon eagle), and others were unofficial division mascot badges, like the greyhound badge of the 115th Panzer division ("Windhunde"-Greyhounds)
Prost!
Brad