You would probably need to agree this with the developers. I am sure most Indian developers would opt to be paid in US instead of their local currency, so maybe they wouldn't mind being paid in SA rands. It depends on which direction the exchange rate is declining. If you can negotiate to pay in rands that would make it easier for your as your cost predictions become easier, but if a fluctuating exchange rate makes their revenue reduce, you could have a problem going forward and lose the supplier, and this type of work is not the type that you want to switch suppliers on a regular basis.
Generally sellers of goods and services prefer to be paid in their local currency, unless their economy is in the toilet.