I'm not sure if FRN are lawful money just because you redeemed a paycheck. Correct me if I'm wrong but you get a pay check from an employer. You go to employers back and non-endorse the check redeeming it for lawful money. The employers bank then gives you FRN in return. You merely deposited the check which was then redeemed in lawful money for the employers bank but not for you. you authorized the bank to go to the treasury bank to redeem the money. but they merely gave you FRN in exchange. the FRN are not really lawful money. wouldn't going to your back and depositing these FRN on a slip saying depositing lawful money be fraud as the FRN notes aren't lawful money? The FRN haven't been redeemed just the paycheck.