I found it challenging to get Currency working on OpenSim Next Gen so here's what I figured out. It seems to work for me but if there's a better/correct way please let me know!

This assumes you 'git' cloned OpenSim-Tranquillity and did a checkout for the master branch.
then build using 'dotnet build --configuration Release' etc...

I put my build files in /opt/opensim/
I use 'screen' to run Robust, OpenSim and MoneyServer 

copy MoneyServer.ini.example to MoneyServer.ini (same location as your OpenSim.ini) and edit the file to your likings

$ cp /opt/OpenSim-Tranquillity/addon-modules/OpenSim.Region.OptionalModules.Currency/bin/MoneyServer.ini.example /opt/opensim/MoneyServer.ini
$ cd /opt/opensim
$ vi MoneyServer.ini

edit OpenSim.ini and Robust.HG.ini (or Robust.ini depending on your setup)
OpenSim.ini:    EconomyHelper = "${Const|BaseURL}:8008/"
OpenSim.ini:    CurrencyServer = "${Const|BaseURL}:8008/"
Robust.HG.ini:    economy = ${Const|BaseURL}:8008/
 
Start MoneyServer

$ screen -R MoneyServer
$ cd /opt/opensim
$ ./OpenSim.Server.MoneyServer

Then you should see a MoneyServer prompt

MoneyServer # help all
...

MoneyServer # stats show
CONNECTION STATISTICS
Client logouts due to no data receive timeout: unknown



Heap allocated:  2MB     allocation rate (last/avg): 0/0MB/s
GCTotalCommited: 0MB     GCTotalAvaiable 64128MB         GCHMthreshold 57715MB
Process memory:      Physical 79MB       Paged 0MB
Peak process memory: Physical 81MB       Paged 0MB

Total process Threads 15

I think you start MoneyServer first, then start Robust and OpenSim in that order