Refusal to issue credit

I have been a customer of Debra-Lee Boutique, Carlswald centre in Midrand, JHB, South Africa for 5 years now. Last year I ordered a pair of pants and paid in advance, and was later informed that they could not make the pants. I was advised by the shop assistant I could get an item for the same price, and the credit would be valid for a year. The credit was recorded on the invoice. I do not have the credit note as I did not get it, but everything that transpired is on the invoice, including the credit note number. Today the owner of the shop informed me that he could not help me, because I do not have a credit note, which due to the negligence of his shop assistant (apparently no longer works there). I am a very disgruntled customer because I have purchased so many items from this shop over the years. I am in the fashion business and would never treat a customer like this, especially when there is proof of the transactions on an invoice. I will be taking legal action.