Yes. This would be legal, but then again, IANAL (I am not a lawyer). If you replace the word ‘currency’ here with the word ‘store-credits’ or ‘loyalty-points’ the whole scenario doesn’t look so bad after all.
The condition here would be that the issuer of this currency, i.e. you, can issue the currency (as much as you like), however, it can only be bought, sold and redeemed at your store, i.e. your business is the only one that can accept this currency. This would make it a closed loop system and be totally legal.
If these store credits can be used elsewhere, for example at Starbucks or another store, then suddenly your currency is not closed loop anymore. It is an