Need assistance in returning the items in the same shop

Team, back in 2016 I bought 2 diamond nosepins and I was told can exchange anytime and I was not told that they will give 80% of the amount and I agreed that after seeing the invoice. Now when I want to exchange I noticed the same item is double what I paid and I decided to sell or in other words to return the item with the same price what I paid in 2016 . Few employees told will get 70% of that amount they won�t charge according to the market price even I agreed that. The employees asked to check with the owner which I did and requested he the owner with a high note denied stating can�t give cash as there is no cash. I checked with other jewellery shops they said the place where we got they are the only people who can buy. Please suggest I need to sell this and give them a lesson.