Currency Exchange

I think your 20000 cedi note is from the old series (with Ephrim Amu?). It's worth 2 new cedi, and the exchange rate is about $1.40.

However, I don't think you can exchange it anymore because the old cedi has been demonitized.

very good that a member correctly pointed out the exchange rate.

warm wishes.

The Krause catalogs are usually good for telling you when a currency has been re/devalued but in this case they don't mention it. A good indicator certainly is when you use a foreign exchange rate program on the net and you get a $10,000+ result. In the case of Ghana this must have happened in 2007 (when new notes were issued) so in fairness they may not have had time to do an update.
If you don't have a catalog then try get a ballpark figure from any recent sales of similar notes from, say, ebay. Or... just ask here!

True enough. The highest face-value note in circulation anywhere today are the $10,000 notes from Singapore and Brunei; those are worth US$7,630 at the moment, so anything above $10,000 is always going to be bogus.