CargoPayment required cargo type to be set as state via SetCargo(). This was error prone as CargoPayment is per consist but cargo type can vary per vehicle part. Additionally if SetCargo was not called then the default "uninitialised" state was cargo slot 0, passengers.
Instead of trying to make sure it is set correctly, remove cargo type from CargoPayment and always pass it explicitly to the PayTransfer/PayFinalDelivery methods.
* cheating by starting to unload and after getting paid rushing to the depot to get sold (or unloading, loading and getting paid again for the remainder)
* cargo being dropped onto a station at the moment a stockpiling industry doesn't accept it anymore
* industries getting cargo that has not been unloaded yet and subsequently dumping it back on the station in one go
Note: you will now get paid after the unloading has finished, so you'll have to wait a bit longer for 'your' money.
Peter Nelson