#Mulder is was a New England fancy boy who was forced to go to etiquette classes and escorted girls to cotillions

[sagan-starstuff]

I think it starts because Mulder is canonically a bad tipper. Scully is from a military family of 4 kids, not a fancy Martha's Vineyard trust fund kid, she probably waitressed in college and is pissed when she notices this fact about Mulder. So she tries to grab the bill as often as possible so make sure their server gets a decent tip, especially when it's on the FBI's per diem. Mulder doesn't pick up on this aspect of it, and turns it into a silly game of trying to pick up the bill for each other. This goes on for years until one day she snaps at him to at least leave 15% for the poor teenager who had to put up with them talking about ritualistic mutilation and murder in the diner.

He gets better about it after that, eventually. She still subtly checks the bill and hides some singles under a plate when she feels the need.

i bet scully and mulder are absolutely INSUFFERABLE when it comes to trying to pay for dinner or drinks. i just know they are playing a game of 5d mental chess to try and make sure they can get the bill before the other does.