All of French West Africa shares a common currency, the CFA, tied at
50:1 to the French Franc. In Paris you can convert unlimited amounts of
Francs to CFA and back, thus giving these countries a hard currency,
and French colonials a place where they can still do business as
always.
The Anglophone countries (Nigeria, Ghana, The Gambia) all have their
own currencies, with a corresponding rate of inflation, but (it seems)
very serviceable. All countries have exchangeable currencies, with
Bureau de Change at every street corner and ‘change money?’ a part of
the greeting exchange with tourists.