Hi Sara,
Hope you are doing good.
I am also working around real time payments which may related to your scenario.
Please find the bellow threads which i am looking from solution end.
O Data service interface - Function modules for Real time Payments.
I just want to know, is you are dealing with same real time payment scenario?
If Yes/No...can you share your understanding on this external cash desk and real time payments.
Regards,
Hari