Please integrate a method for Voiding Cash Transactions. Include a pop up window or screening process to ask user to verify Cash return/ balance if needed. At least incorporate it mid sale when transactions are split.
There was an example when we paid half card and the other payment was missing. I then had to proceed with cash to make any other correction possible. I then had to return funds and cash. This affected our sales tracking system and manipulated margins (Transaction counted as a Return and affected margins)
Another Suggestion is to keep Card transaction pending until the Sale has been finalized.