Generally, it is as you say "Is a taxable supply identified the earlier of payment received or invoice issued?", in that it is the earlier of.
But a deposit is frequently excluded. Have a look at this document;
https://www.saica.co.za/integritax/1...s_received.htm

I am not aware of any legal issues regarding the pro forma's and the currency used. However as Busfact mentions, you have to maintain your records in the currency of country of location. It is normal to reflect the home currency. I suppose nothing stops you from reflecting the rate too, although for a buyer it is different than to a seller in the absence of forward cover.

Furthermore, you have to account for exchange rate differences. This means the difference between the rate at date of revenue recognition versus the rate at date of receipt of the money (a realised gain or loss), and date of revenue recognition versus reporting date in respect of unpaid sales (an unrealised gain or loss). The IFRS requirement for the time of revenue recognition is pretty complex and is being revised. You should get your auditors / accountants to advise you. Realise that it will affect when you reflect the sale, which will in turn affect the calculation of the currency gains or losses.