It's quite straightforward. If you don't already have an Offline Card button you can change your existing one.
Go to Program Mode - Option 3 - Function Keys - Finalize Keys
Find your Card Payment button and go into the settings for it. Change the key type from EFT to CASH, and change any other settings toy require while you're there.
Now when you use the Card button on a sale it will simply finish the sale rather than trying to send the payment to your card machine.
If you need any further help just let me know.
- Steve