The business dress code in Ghana is a sliding scale that depends how familiar you are with the person you are interacting with.
First interactions at someone else's office - you are expected to be appropriately suited and tied. Government offices, always. Obrunis are judged for going too casual, so better to err on the formal side when in doubt. Going out for dinner, a shirt without tie is perfectly acceptable. Never jeans, even for a casual business dinner.
Once you've been there for a while, dressing down is also expected. I spent almost 8 years working there and my everyday "uniform" was a short sleeve white shirt with dark slacks (no, I wasn't an LDS missionary!). That included for very regular meetings with Government departments, banks and even the President (the current President abhors suits and won't wear one except for formal occasions - I can't remember ever meeting him in a suit, going back to when he was VP and even before).
I second Stuart's suggestion of getting a nice traditional shirt and wearing it on Fridays if the office has a "traditional Friday" policy as most do. Nothing builds ties better with your local colleagues. You'll know its a success if they start asking you where you got it from and start giving you tips on where to buy the next one from!
