Repair costs

We bought a wedding ring at Sterns Alberton in 2016 July or August , currently the ring is missing a stone. I returned it to Alberton when I went back to check the progress on the repair I was always turned back being advised they cannot find my ring. On my last visit i was advised I need to make a payment of R1500 for repairs. This was only communicated to me after several visits to the store. The ring still has a guarantee, why was I not informed about the payments when I booked it in with them. I will not pay the said amount as the ring has a guarantee and no one informed me of the costs involved.