read this paper (maybe)?

https://www.persee.fr/doc/cea_0008-0055_1985_num_25_97_2185

It's old but one of the best summaries I've seen on the economic dynamics of that region and during that era.

Igbo currency discussion starts on page 59. Some takeaways just in case I'm wrong (about you reading it before) and/or tl;dr :

-The monetary system, like the Igbo system of governance, was mostly decentralized i.e. no central authority, monarchy, etc dictating rules of currency exchange or value

-Cowrie shells (which originate from a type of snail native to the Indian Ocean (i.e. link to Arab Slave Trade) and were first introduced to Western Africa via Trans-Saharan trade, then Igboland specifically via trade around the Niger river basin) were more ubiquitous than metal currencies (archaeological evidence points to limited quantities manufactured by Igbos via a labor-intensive process before introduction of cowries, but later-generation metal curriencies were also imported by Europeans during the TAST)

-Imports were priced according to whatever currency was used at a specific port, forming different monetary zones. I find this to be an interesting contrast to how we normally think about currency... It's like travellers having to use currency A if they fly into JFK, and currency B if they fly into LAX; but New York/JFK only using currency A because it gets so much traffic from country A, and Los Angeles/LAX only using currency B because it gets so much traffic from country B. Weird.

-Early in the TAST, Europeans preferred to trade with their exports instead of currency whenever possible to undercut Africans (it was more expensive for them to use currency). But as they industrialized, currency became easier/cheaper to transport(in the case of cowrie shells) and manufacture (in the case of metal currencies) and they switched to trying to flood the Igbo markets (probably to drive their value down and, again, undercut Africans). Is this the currency manipulation you're alluding to?? Also noteworthy that Igbos responded to these tactics by a) refusing to accept the newer-generation shells and metal rods/bars in their markets (to keep rates stable) and b) withdrawing surplus currency out of circulation by repurposing them as jewellery and other symbolic uses (i.e. quantitative tightening, but also the likely origin of cowrie, bead, and bracelet/anklet accesorizing that accompanies traditional styling even in today's time).

- Just to tie this back to the OP, I find the section characterizing Igbo market networks and their admin (starting from page 63) relevant to countering Nwaubani's claims too. Because it points out how local policies set by the women collectively leading these village market councils included approving and disapproving which other towns/village groups could sell goods in their respective markets and what could be sold. I think this gatekeeping would have significantly limited the number of markets profiteers (i.e. raiders and traders) could use as venues for their trafficking, since slavery and other associated sacrifical practices were not customary across all/most Igbo groups. Furthermore, profiteers had to rely on personal/kin relationships to gain market access in the first place which would have limited the venues available to them even more.