Here's a news article from April 2014,
African Monetary Union Stirs Criticism of France
A hoard of cash sits in the Bank of France: $20 billion in African money held in trust by the French government and earning just 0.75 percent interest. Now economists and politicians from 14 Central and West African countries say they want their funds returned and an arrangement dating back to the days of France’s colonial empire ended.
France holds the money to guarantee that the CFA franc, the currency used in the 14 nations, stays convertible into euros at a fixed exchange rate of 655.957. The compulsory deposits started more than half a century ago, when the then-colonies had to place all their financial reserves in the French Treasury. The deposit requirement has dropped over the decades: Today the African members entrust 50 percent of their reserves to Paris.
This article matches the claim which is quoted in the OP, in a couple of ways, notably that:
- There are 14 African countries involved
- They are putting a large fraction of their foreign reserves into France.
IMO these two matches are sufficient to show that the news article in this answer matches the notable claim identified in the question.
However this article doesn't match (i.e. it denies or contradicts) the claim in other ways,
- It's a fraction of their reserves: it's not a 'tax' or an income tax
- It says 50% of their reserves, not 85%
- It says "a hoard of $20 billion" not "effectively 500 Billion dollars every year"
- No mention of assassinating African leaders who don't comply
The 14 countries in question are presumably those whose currency/money is the West African CFA franc (8 countries) and Central African CFA franc (6 countries).
At least one of the articles referenced in the OP also mentions colonial-era debt. Repaying debt is not the same thing as keeping foreign reserves to stabilize/guarantee currency exchange rates ... but "debt" is not the quoted notable claim.
As for whether countries "have to" do this, IMO the following is evidence that the various countries are able to leave this scheme and/or that they may choose to join it:
CFA franc - Changes in countries using the franc
Over time, the number of countries and territories using the CFA franc has changed as some countries began introducing their own separate currencies. A couple of nations in West Africa have also chosen to adopt the CFA franc since its introduction, despite the fact that they were never French colonies.
- 1960: Guinea leaves and begins issuing Guinean francs
- 1962: Mali leaves and begins issuing Malian francs
- 1973: Madagascar leaves (in 1972, according to another source) and begins issuing its own francs the Malagasy franc, which ran concurrently with the Malagasy ariary (1 ariary = 5 Malagasy francs)
- 1973: Mauritania leaves, replacing the franc with the Mauritanian ouguiya (1 ouguiya = 5 CFA francs)
- 1974: Saint-Pierre and Miquelon leaves for French franc
- 1975: Réunion leaves for French franc 
- 1976: Mayotte leaves for French franc 
- 1984: Mali rejoins (1 CFA franc = 2 Malian francs)
- 1985: Equatorial Guinea joins (1 franc = 4 bipkwele)
- 1997: Guinea-Bissau joins (1 franc = 65 pesos)
I don't know who is responsible for choosing the exchange rate (which almost never varies).
CFA franc - Exchange rate
The CFA franc was created with a fixed exchange rate versus the French franc. This exchange rate was changed only twice: in 1948 and in 1994.
The value of the CFA franc has been widely criticized as being too high, which many economists believe favours the urban elite of the African countries, which can buy imported manufactured goods cheaply at the expense of farmers who cannot easily export agricultural products. The devaluation of 1994 was an attempt to reduce these imbalances.
Here are a few quotes from the Bank of France on the subject.
Présentation de la Zone franc
La Zone franc regroupe 14 pays d'Afrique sub-saharienne1, les Comores et la France. Bâtie sur les liens historiques étroits qui unissent la France aux pays africains, la Zone franc est issue de la volonté commune de ces pays de maintenir un cadre institutionnel qui a contribué à la stabilité du cadre macroéconomique :
La coopération monétaire entre la France et les pays africains de la Zone franc est régie par quatre principes fondamentaux : garantie de convertibilité illimitée apportée par le Trésor français, fixité des parités, libre transférabilité et centralisation des réserves de change. En contrepartie de la garantie du Trésor français, les trois banques centrales sont tenues de déposer une partie de leurs réserves de change sur un compte dit « d'opérations », ouvert dans les livres du Trésor.
Le fonctionnement du compte d'opérations a été formalisé par des conventions signées entre les autorités françaises et les représentants des banques centrales de la Zone franc. Ils fonctionnent comme des comptes à vue ouverts auprès du Trésor français et sont rémunérés.
La coopération entre la France et les pays de la Zone franc se traduit notamment par la tenue, deux fois par an, des Réunions des ministres des finances des pays de la Zone franc.
The Franc zone joins 14 countries of sub-Saharan Africa, the Comoros,
and France. Built on the historic ties which unite France with the
African countries, the franc zone is born from the common desire of
these countries to maintain an institutional framework which has
contributed to the stabiliy of the macroeconomic framework:
The monetary cooperation between France and the African countries of the franc zone is ruled by four fundamental principles: guarantee of
unlimited convertibility brought by the French treasury; fixed
exchange rate; free transferability; and centralization of the
exchange reserves. In counterpart to the guarantee of the French
Treasury, the three central banks are required to deposit a portion of
their reserves in an "operations" account, open on the books of the
The functioning of the operations account has been formalized by conventions signed between the French authorities and representatives
of the central banks of the franc zone. They act like open accounts on
the French Treasury and are remunerated.
The cooperation between France and the countries of the Franc zone is notably made manifest by holding, twice a year, the Meeting of the
ministers of finance of the countries of the franc zone.
A new African Monetary Union is being discussed or proposed for the distant future e.g. the 2020-2030 kind of timeline.