Credits system is already handled by IP.Nexus. You can setup minimum and maximum amount of credits Customers may purchase. Those credits can be used for any further transactions.
1 Credit = 1$. I did not see anywhere a way to specify amount of credits given for the single unit of specified currency.
You can setup currency and override locale in Store & Client Area Settings area of Nexus.
I haven't checked if there is a dedicated language bit for this specified area of the Nexus to change the "currency" credits are displayed with, however it would be worth to check language management in your ACP.
Oops! Google Chrome could not connect to www.invisionpower.com
Marketplace is loading ~9 seconds
Tracing route to invisionpower.com [184.108.40.206]
over a maximum of 30 hops:
1 <1 ms <1 ms <1 ms MitraStar.Home [xx.xx.1.254]
2 51 ms 16 ms 16 ms xx.xx.104.1
3 17 ms 17 ms 17 ms xx.xx.59.253
4 23 ms 21 ms 25 ms tenge-6-1-1.core2.dbn.core.eircom.net [xx.xx.253
5 25 ms 21 ms 21 ms tenge-2-2-1.core2.bdt.core.eircom.net [xx.xx.252
6 21 ms * 20 ms ge0-0-0.peering1.csg.dublin.eircom.net [xx.xx.24
7 * * * Request timed out.
8 131 ms 131 ms 131 ms ae-12-12.ebr2.Paris1.Level3.net [220.127.116.11]
9 131 ms 131 ms 131 ms ae-44-44.ebr2.Washington1.Level3.net [18.104.22.168
10 133 ms * 134 ms ae-46-46.ebr2.Washington12.Level3.net [4.69.202.
11 131 ms 132 ms 130 ms ae-6-6.ebr2.Chicago2.Level3.net [22.214.171.124]
12 131 ms 131 ms 131 ms ae-202-3602.edge2.Chicago2.Level3.net [4.69.158.
13 131 ms * 131 ms EASY-ONLINE.edge2.Chicago2.Level3.net [126.96.36.199
14 138 ms 137 ms 137 ms lw-core4-te91.rtr.liquidweb.com [188.8.131.52]
15 135 ms 135 ms * lw-dc3-dist14.rtr.liquidweb.com [184.108.40.206]
16 139 ms 137 ms 137 ms 220.127.116.11
17 138 ms 138 ms 137 ms corp01.lw.ipslink.com [18.104.22.168]
Not a really big money I'd say. Anyway, they just keep using Nexus for maintaining their sale where Nexus itself is not a greatest sale maintainer solution at the very moment. If it would be more flexible then for sure the payments and charges would be maintained other way.
I experienced this issue myself with Nexus while trying to configure easy and quick sale and purchase maintaining. As soon as they will rebuild Nexus then you can expect improvements on their sales in relation to charges. They are simply limited with Nexus possibilities ;) as far as Nexus is great app it requires the biggest workload to be done over among all the other IPS.Apps at the very moment - just my personal feeling about where IPS does not fulfill my needs.
I remember few times I cancelled some of my app by the mistake, however filled a ticket and they made it up for me without a charge - so they aren't heart-less :D