If you can confirm that you were overpaid, then you should pay them back. You also asked whether they can withhold your paycheck to take back the money owed. That's not as clear. If you did the work to earn the pay, then they should pay you based on your work and take the appropriate deductions for Medicare, etc. that they normally would take. You two should work out how to pay them back the overpaid commission separately.
If the withholding resulted in the wages falling below minimum wage, there may be a violation of the Fair Labor Standards Act and Florida law requiring payment of at least minimum wage.
