You say that, but I didn't hear a rule against it with だって

Grammar Explanation: That said with だって

Standing back to back, Sacchan is slightly taller than Yui. Yui questions whether Sacchan is standing tip-toed, which Sacchan denies, but Yui looks and sees that Sacchan is.

さっちゃん:: 「だって背(せ)のびしちゃダメなルールって聞(き)いてないし」; “You say that, but I didn't hear a rule against tiptoes.”; Literal: “Because I haven't heard of a rule saying tiptoes are no good.”
結衣(ゆい):: 「背(せ)のびしてもいいルールなんてないもん」; “There's no way there'd be a rule allowing tiptoes!”

Key Points

1だって pushes back with an excuse

Sacchan uses だって to answer Yui’s accusation about standing on tiptoe.
It has a “but” or “because” feel: she is defending herself.

2聞(き)いてないし leaves the reasoning open-ended

聞(き)いてない says she has not heard such a rule.
し implies this is one reason among others, adding a casual argumentative tone.

3The rule phrase is packed into one noun

背(せ)のびしちゃダメなルール means “a rule saying tiptoeing is not allowed”.
〜ちゃダメ is casual for 〜てはいけない.

You say that, but I didn't hear a rule against it with だって

Key Points

See Also