The authority of Koine Greek is BDAG. It defines (Thayer has almost identical results) the κέρμα and itits plural in John 2:15, κέρματα as
(from κείρω "cut short, clip") piece of money, coins (small) change (usually copper) collective singular τό κέρμα ...
BDAG also lists numerous references and other occurrences of this word in secular Koine Greek texts. See BDAG for much detail.
A closely related word from the same cognate root appears in the previous verse (John 2:14) as κερματιστὰς = money changers.
There is no mention of this being distinctly African or anything else. Indeed, the African kingdom of Kerma (and its language) had ceased to exist more than 1000 years before Christ, before even ancient Greek (the predecessor of Koine Greek) was even spoken anywhere. Thus, any connection to this ancient kingdom had ceased to exist well before Jesus walked the temple courts.