Who's going to be your counterparty and why would they give you the "official" rate? The whole reason why there's an "official" rate and a "black market" rate is that the government has mandated one rate, but the broader market disagrees and has agreed on a different rate. The two never converge because the government requires you to jump through hoops to get the official rate, so people can't arbitrage the difference.
> Banks I assume? You would get the market rate, not the official rate.
So where's the money making opportunity? You're short selling at the market (ie. non-official) rate, and have to buy back at the market rate. The only way you'll make money is if the market rate drops even more, but whether that happens is uncertain. This just sounds like run of the mill currency speculation.
