I was in Nigeria for about two weeks back in 1994, doing business. To a large extent, the answer to your question is that dash (the Nigerian term for “cash under the table”) is just part of the business world.
We were told by our clients to resist any efforts made at collecting bribes. When we entered the country, the guy at immigration gave us minor troubles, we had to provide additional documentation that was not legally necessary. He was clearly looking for us to grease the skids; we just played ignorant. The guy at customs opened my suitcase and asked, “Did you bring any presents for me or my friends?” Then, since we hadn’t paid up, there was an armed guard who stopped us at the airport exit. A crowd gathered, and a couple of them whispered to me to just give him money; fortunately, our client had met us. It was really funny if it wasn’t so scarey: she was about 4’10" and he was well over 6’, built like a football player and loaded with weaponry. She barely came up to his navel, and she started yelling at him: “What kind of impression are you making on these visitors to our country? You know better than that!” and chewed him out. He backed down.
It was pretty much that way every step. Anything we bought was bartered. Salaries, certainly at executive levels, are designed to (legally) evade taxes, but the laws are unclear, so they get away with lots. In terms of infrastructure like roads and such, at that time anyhow, there wasn’t any. The phones (we were staying in the fanciest European-owned hotel in Lagos) didn’t work half the time; the roads were so full of potholes that there might as well not have been any.
The reasonably well-to-do and the rich live behind gates, high fences, barbed wire and hired their own security guards. There were hundreds of people on each street corner, presumably homeless, barely clothed; driving by, I could see many of them sick, covered with sores, etc. I was told (by a very well-to-do accountant) that the oil companies destroy the villages and the people flee to the cities, looking for work, food, etc. But the cities are flooded with this population expansion that they can’t handle.
I’ve been in several “third world” countries, but this was one of the saddest. On the plus side, I’ve got enough stories for many, many cocktail hours.