Wrongfully terminated without progressive discipline

I was terminated without notice today, without any form of progressive discipline, for what I consider to be an error in judgement while executing my retail job duties for The Bombay Co. Recently I completed a *very* large retail sale, and in the process of securing it I gave a customer a discounted piece of furniture as part of that sale. The Bombay Co contends this was grounds for dismissal and also contends I lied about why I did it (which gives them greater grounds for the dismissal). Given my staff position (upper) and responsibilities, approval rating with other managers staff and clients (immpeccable) I feel I have been wronged.

Do I have grounds for wrongful dismissal since:

1) Have an immpecable employment record. *Several times I had been asked by *this* company to join their management; and

2 I was never offered any progressive disciplinary actions or options.(Normal recourse is to "write up" individuals first etc.) I was terminated in the course of a private 15 minute conversation "out of the blue" today.
I can provide references as to my work ethic showing I was an employee beyond reproach and always worked in good faith.

Thank-you
A.T.