In case you try to confirm these figures via Google, the exchange rates for the amounts I stated are based on the true (i.e., black) market rates (740 Naira/$1) instead of whatever Google shows.
Are there rules and regulations preventing access to the official forex markets?
The Naira's value plunged against the dollar. The Central Bank still insists the Naira is worth around 453/$1 while the free market insists it's much worse.
So, the Central Bank continues to advertise N453/$1 but you can't swap your Nairas to dollars - only at black (i.e., free) market dealers who will give you the true market rate.
Come to think of it, that's true of all "black markets". The only thing that makes a market "black" is the existence of some ridiculous official alternative.
[1] For example: when I land in Japan, I don't get Yen from the JCB -- I get it from a private entity who sells it to me for dollars, usually at a worse than officially stated exchange rate. The Travelex booth at the airport is literally a private market for cash. When I swipe my credit card in a foreign country, the intermediary banks are more than happy to sell me currency, on the spot, for a small fee.
Check Argentina -- their "blue rate", used by hotels and other cash-based businesses on site, is often twice as favorable as the official rate that you get with plastic.
If a black market exists, that means the "official" markets are corrupt.