Go to Manage | Database | Fields.
Find Date Deposit Paid field and double click it ot open the Field Options dialog.
Select the Auto-enter tab.
Select the calculation option
and enter or select the name of your Date Balance Paid field as the sole calculation term. Click OK.
Leave the "do not replace existing value" check box selected and then entering a date into Date Balance Paid will automatically copy that date to Date Deposit Paid if the Date Deposit Paid field is empty.