I've not worked for 'the man' for a long time, but I'm fairly certain all of my employers would have deducted the outstanding balance from the final cheque because there is little point in employing people to make sure that people are paid the correct amount, with the correct deductions made, if they aren't doing that.
I'd be amazed if the software they use for payroll doesn't automatically make the deduction when they tick fill in the 'Leaving date' box.
Keeping it is theft from the employer, and tax fraud, so theft from everyone else.