Generally speaking--
To make leather softer:
add moisture - temporary results
add oil - usually long term results
To make leather harder/stiffer:
remove moisture - temporary results - humidity changes results unless leather is sealed
remove the leather oils
To remove moisture, apply heat, too much heat may burn leather or cause shrinkage
To remove oil:
rinse in a solvent - alchol, acetone etc. denatured alchol, or isopropol are most controlable
wash in soapy water (not detergent, too harsh, soap)
boil in water, the longer you boil it, the harder it gets. In days of old they used to boil leather to make armor!
Saturating leather with paste wax can soften real stiff leather or or stiffen real soft leather. Apply with a rag or sponge, heat with a hair drier or heat gun until the wax melts at which point it will be asorbed by the leather. Repeat until the leather won't absorbe any more wax. DO THIS AFTER APPLYING DYE NOT BEFORE!
The above being said, try these on scrap before using on a scabbard.
Hope this helps
Jim Arbuckle
ABS JS