This is what your internal procedures should detail. I would be asking for very detailed documentation which details who the Nigerian company (including their BofD and Executive Management) is, what products they distribute, a list of their customers, a selection of invoices to review and how long has your customer been doing business with this entity.
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"100 victories in 100 battles isnt the most skillful. Subduing the other's military w/o battle is the most skillful." Sun-Tzu