A Dollar Shop

I had an informal credit slip of $20 that was given to me by an employee at this store back in June of 2012. I've seen this employee many times before at this store. I always buy this one product which they have been out of so I've been in about 3 times just to request that they order it. I went in about 2 weeks ago and found that it's available. So I purchased $22 items and wanted to use the credit I was given. The owner refused to accept the credit, and began to argue with me when I gave my reasoning for using it. Then his daughter and son-in-law also join in the conversation and made the situation extremely embarrassing for me, the customer. I felt them ganging up on me. I was told by the employee who gave me the credit, [redacted], that the owner is his uncle and my credit was verified with both of them in October when they were both in the store when I visited last. Now the owner said he cannot honor the credit, he know nothing of it and that he has never seen me in his life! He claims that he found [redacted] stealing and has been fired. But what does that have to do with me? I patronize this store generally every 1-2 months and had seen everyone who worked there including the owner's wife. I had no choice but to pay for the products with my credit card without being given the credit.It was a terrible experience and clearly a poorly run small family business which has no professionalism. I have made purchases there before which are suppose to be a dollar and not priced individually but this owner charges more, ie. $1.25 and doesn't give out receipts!This store has poor business practice. While I was in the heat of the argument, another customer was rung up and also returned to ask him how much the item was because he charged more than a dollar!