Eyewear cancellation

I'm very upset. Last month 27th April I went for the first time to them for glasses. After the eye test I chose a frame and they told me I can pay off in 3 months but I have to pay half of the amount before they can start processing. So I gave a deposit of R500. Today I decided that I won't be able to continue with the payment in 3 months time because it's too expensive R4607. So I've asked them to cancel and explained my reason. So I've been told I won't get my money back since the eye test is R600 I still owe them R100 because of the cancellation. If I was going to continue with the glasses they was going to give me discount of R300. They never explained to me about cancellations and the eye test is R600, I mean they just decide to tell me that now only because I want to cancel. It's not right to just take R500 from a client without informing the client before time about cancellations. And I've even been told I won't get my eye test description until I pay the R100😡😡😡What am I to do now? Because I can't just let my money go like this so I have to take this matter further. 😡😡😡