I'll leave it to an expert re the holster
but any leather gun items I have I put on leather dressing
over a period of time and it gradually softens and moisturises
the leather. Then if you rub it lightly with a good cloth
it will get some shine back to it anyway.

Whatever you use (leather dressing), try it over a period of
a week on a small, out of the way area first just to make sure.