I agree that it's a borderline issue. However, seeing that in .de the customer has explicitly NO right to ask for a refund or return when buying PMs (because the items are subject to exchange rate fluctuation), I would further argue that here the same is the case.
So either see that the guy gets the Rand or refund the VALUE of the rand.
Turn it around - you actually deliver, the price falls, you will NOT get 1605USD for your coin, but only 1500USD.
I agree that the circumstances are different, but the principle applied is the same.
Disclaimer: IMHO, since IANAL.
It's not a borderline issue.
It's a very simple issue. Refunding the "value" of something in different (and lesser) denomination than you gave is completely crooked. We're not talking about
returns. We're talking about
refunding the failed purchase of an item that never shipped. I don't understand why this is so hard to appreciate and comprehend. This is basic business and accounting 101.