Been thinking about it all day.
OP had a credit card that gave her a discount at a restaurant so everyone would pool together for lunch and send the cash to her the same day.
Except for one coworker, who would wait until the last day the credit card was due and pay in cash. She tried to say the OP was somehow making money off them as she had a whole month to pay her credit card.
Eventually the coworker stopped paying all together...
I really want a satisfying end.