whats a good white lie if youve been fired for arguing with a customer?

i recently got fired for arguing with a customer, their regulars and always come in looking mad or their very rude, anyways my shift was about to end and the shift manager at the time said i could clock out but before clocking out i decided to take the customers order before i left cause they looked (again) mad but i didnt know that my manager had closed my register and i said i would be able to help them at the next register and when i touched the screen i noticed it was closed and i apologized for not knowing and they got bck in line and asked my sister if she had a problem with her, my sister got the manager to deal with them and he did, then went back to what he was doing and the customers kept looking over at us and i waved hi to them and then the girlfriends boyfriend flicked me off and then came up to my sister asking if we had a problem with his chick then started to yell and curse and say racial slurrs saying that we were low life mexicans working at mcdonalds and he makes more than us and me and my sister were arguing back at them and when he flicked me off i flicked him off back. A few days later i was fired for flicking the customer off and arguing with him. My general manager and i have a very good relationship, i was a hardworker and i was reliable and i went above and beyond to get work done and to help others, i just made a mistake. So my question was whats a good white lie if youve been fired for arguing with a customer and what should i put on application for reason for leaving and when it asks to explain why i was fired?
My manager already said he would put in a good word for me if any people call him regarding me, but im unsure what to tell interviewers cause i don't want to say something different from my manager.