I would like to ask a question concerning possible Wrongful Termination?

I was forced to sign a Letter of Resignation from a Union job of nearly seven years regarding policy concerning the rental of automobiles. I say forced, because all my benefits were held until I agreed to sign, such as insurance and pay. I was never given any prior warnings over the rentals and had always followed the same procedure of obtaining prior authorization before each rental expense. The claim was that I received an allowance for the use of my vehicle that was added in as part of my salary each month that I used also. No one ever explained that I was to tell payroll to stop putting the allowance on my check. I had to turn in all my expenses and obtain prior approval before each rental. The Employee manual states that the punishment for unauthorized expense is loss of credit card.