I believe you're referring to the CFA franc. This is not a tax, as countries can leave this agreement at any time, but it does require member countries deposit a certain percentage of their currency in the French national bank. In that sense, those countries are "obligated" (if they want to stay in the program) to pay France a certain amount, but it's not an annual payment as a tax would be, and it's arguably mutually beneficial as it guarantees a fixed exchange rate.

As the other commenter mentioned, the reason they have this agreement is at least partly because they gained independence diplomatically rather than through war. But it's also worth noting that there are countries using the CFA franc that were never French colonies.