Hi. You need to contact sentry for the master pass associated with that safe serial. You don't need a proof of purchase but you need to tell to them you are the new owner with a notarised form:
They ask a service fee of 20$ for that (not counting the authentification of the form).
Sorry that i can't do more online (playing with wheel and pin would be a legality issue from from my side). You can also bring it to a locksmith, those are not that hard to crack the combo.
You can always ask for a refund on the deposit if you are not satisfied. This also mean that the company would not pay me even with a good rating that said.
If you want a visual aid to open it when you have the combo, look at this link: