I was hired at Canadian Tire in May 2011. Since my hire, I…

I was hired at Canadian Tire in May 2011. Since my hire, I was promoted 3 times. My last promotion was Head Cashier, not a day went by whereby I was not praised on my good work. I absolutely loved my job but 6 days after my last promotion I was fired and escorted out the door, accused of fraud and theft. I did something that I know was rightfully wrong, I took a return from my fiancé without a receipt, something that is always done at this particular store except that you are not supposed to aid any family members and not only did I take a return from my Finance, but I took a return of merchandise which my fiancé omitted to tell me was an item that was not necessarily purchased at CT. I thought nothing of it as we sell this particular merchandise. My fiancé then admits to me that our landlord gave him the particular merchandise in question and said do what you want with it.When my fiancé came into the store; he had said to me that landlord had purchased the item at Canadian Tire. I contacted the landlord myself who confirmed that she did ok'd him keeping the merchandise and doing whatever he wanted with the merchandise but never told him she purchased it at Canadian tire, in fact, she told him she couldn't recall. I admitted to my employer my wrong doing and apologized but they would not hear any of it, in fact, they are now accusing me of stealing gift cards etc., I HAVE NEVER, NEVER STOLEN FROM THEM, my only wrong doing is this one incident, now they are threatening me with a lawsuit. What can I do?