Amount eaten with 〜分

Grammar Explanation: Amount with 〜分

Sacchan and her friends are tasked with going around and selling excess bananas from her mother’s fruit shop. They offer a banana to a shop owner, and when he eats it, Sacchan says he needs to pay for it. The owner realizes he fell for their trap, but offers to buy a bunch.

琴葉(ことは):: 「私(わたし)たちが食(た)べてる分(ぶん)も買(か)うといい」; “You should also buy the amount we're eating.”; Literal: “It would be good to also buy the portion we are eating.”
おやじ:: 「まったく。かなわねぇなぁ。わっはっは」; “Good grief, I'm outclassed. Wahahah!”; Literal: “Honestly. I can't beat you. Wahaha!”

Key Points

1食べてる分(ぶん) means the amount being eaten

食(た)べてる is casual for 食(た)べている.
分(ぶん) turns that action into a quantity: the portion the girls are eating.

2も買(か)うといい adds that amount too

も means the shop owner should buy not only the banana he ate, but also theirs.
〜といい gives advice.