There are a couple of possible translations, but this time I'd recommend capital contribution which emphasizes it is the property of someone, the seller here.

As per the "-ben", there is no need for the comma in "merteke a torzsbetetje, a Tarsasag torzstokejeben", at least how I can interpret is that the share of the seller is his capital contribution to the equity capital of the company, that is, the contribution is part of the equity, the contribution is "within it" in Hungarian logic, and that's the reason for "ben", the inclusion.