I am having some trouble writing use cases for XML mapping after a certain trigger invoked by the system.

For example, one of the scenarios goes: the store cashier sells an item, the transaction data is sent to Data management system. Now, I am writing a functional design for the scenario which deals with mapping XML fields between our system and the data management system.

Question : I was wondering if some one had to deal with writing use cases or extension use cases for mapping XML fields between two systems? (There is no XSLT involved) and if you used a table to represent the fields mapping (example is below) or any other visualization tool which does not break the bank ?

I searched many questions on SO and here but nothing came close to my requirement.