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.”
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

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.
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.
The 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.