Hold inventory exactly equal to what the customers deposited, be it crypto or fiat. The exchange rate matters only when trading.

For stable coins they have to be backed 100% by reserves of real currency. For crypto - well you just hold the crypto and don’t lend it out. If I give you 1 ETH, you just keep it in the wallet for me to withdraw on my leisure. Simple.

arguably, all the crypto exchanges are proving crypto's a fiat too.