It's a classic roleplaying scenario - you need to buy some stuff, and so you have to bargain with the seller to negotiate a price. The trouble is, some people love to bargain and haggle, but some people really don't. Don't try to force a player who hates haggling into a haggling session where they actually have to roleplay it.
You can set up the situation, but if the player sidesteps the haggling and asks if they can just roll some dice, let them.
If only real life worked this way when faced with the unexpected haggling situation...
Commentary by memnarch (who has not seen the movie)
A 4 is a pretty bad roll. But I think I'd still roleplay it myself if I was in the mood. If you play well enough, you might be able to get a bonus of some kind. Example: "Only a quarter portion? These seemed about as good as the ones yesterday, did something change?" Then the GM can roll with it in directions like: "It's all I've got available at the moment; the rations machine broke for a quarter of the day" or "We needed the parts you brought yesterday. These are the same, but now we don't need them". Then it's a less of a failing on the player's part. It could also lead to different minor plot hooks of some kind if the GM's interested in putting out hooks like that.
Jeez those are some stubby fingers. Unkar must be very good at grabbing things with their palms or appraising things with just their eyes. Or perhaps they're a minion of the town's boss; I can't see much else for why they'd be in charge of the commissary. The quality of the rations looks about what I'd expect for a run down outpost. Better even, as it looks like the packet is still sealed up nicely. It still probably tastes pretty bad though.
And now Rey is out of parts, needs some water both to drink and to reconstitute her food, and wants to leave town tomorrow on a hoverbike that's probably been stolen for parts by now. Half the players are on Team Antagonist, and the other half are scattered about or busy finding a new character.
Commentary by Keybounce (who has not seen the movie)
Possible title: Yes Quarter Given.
Alright, so lets look at the implications here. Rey is not just visiting a random location. She's using a cleaning station, she knows the operation of this location and how the trading works - so this is a place that she frequents for her salvaging. Which makes sense if this is a major trading post - again, the commissary needs to make a profit on the parts brought in by the scavengers, so like any market, there would be a good supply of customers and suppliers. This is like a non-mobile Jawa station, where the customers come to Plutt and the rest of his workforce to acquire parts, and like a "junk store", or "assorted curios" store (serious question: what's the proper name for that type of shop?), there'll always be a lot of stuff that moves very slowly / isn't demanded at any price, and the occasional good thing that goes quickly at any price.
There are probably other people on Plutt's staff. He might be working with three others.
At least Rey does not need a piece of a large wooden stick to help her walk; that would be a quarterstaff. If she were to whack the sand with it, possibly to herd sand worms, she might be a quarter pounder. If she has a room, he might be her quarters master.
I'm sure that at least 25% of the thread comments will have similar bad jokes.
GM: At the commissary you barter your salvaged parts for provisions with the owner, Unkar Plutt.
Rey: I make a Bargaining roll.
Rey: Hrm. 4.
GM: Do you want to roleplay it?
Rey: Will it make a difference with that roll?
Unkar Plutt: What you’ve brought me today is worth... Hmmm...
Unkar Plutt: One quarter portion.
GM: Plutt slaps down a small packet of dehydrated rations.
Rey: A quarter? Really?
GM: He is the quartermaster.