Can't set Currency for Invoices

Hello there,

I am currently working on a function to add AR Invoices via DI api to SAP but we have a problem. When trying to set the currency the program runs into an error (object reference is not set). If I don’t set the currency it works perfectly fine but it then just uses the currency from the Business Partner but that is not what we want.
We are using coresuite customize (version 5.20.22949) add-on to add this function.
I also tried to do the same in Visual Studio C# and it worked completely fine there and I was able to set the currency there so it shouldn’t be a problem with Code but maybe I have to set some kind of setting for it to work in coresuite?
Has anyone an Idea?

Best Regards,

Alexander