1. Spread between buy and sell price. However, certainly there are more people sending money into ARG than out.
2. Service fee (most people do not use promo codes) and actually pay a fee.
3. Interest on the money as they complete the transactions. As they may hold the cash for a few days before they pay out they potentially make a small interest profit.
Finally, it's possible Argentina is not a very profitable market for WU but as long a they are not losing money they want to have boots on the ground in every country.