Select the driver you want to assign the cash to and click Driver Status.
The Driver Status window will appear. You will see under financial summary that there is no cash currently assigned. Click on the "Assign Bank" button.
A new window will popup with a keypad. Enter in the amount of cash you want to assign the driver and click Assign.
Once you have assigned the cash, the "Cash Assigned" field will update with the amount.
Tip: You can specify if a bank is required, and the default amount, in Configuration / Order Types / Delivery / Driver Management column. Also, you should check the configuration in Tax/Tender/Cash - Cash Management to determine where the cash for the driver bank comes from.