AITA for asking what I had to pay !?

AITA?

I’m wondering if I’m the asshole here.

My brother invited me over for dinner, but the invite came through my dad. I never spoke to my brother directly. I agreed to go, but because I’m a bit short on money due to Christmas, my dad offered to pay for my taxi there and back, and said I could pay my share of the taxi when I get paid.
The next day, I got a message in a group chat from my brother’s wife asking if I could transfer £10 for the meal, as they were going shopping for ingredients.
I hadn’t been told beforehand that I’d need to pay, but I understood and thought that was fair. Since I didn’t have the money at the time, I asked my dad to lend it to me until I’m paid.
My dad then told me I actually needed to pay £20. I asked why, because my brother’s wife had just said £10. My dad said he didn’t know, just that my brother told him it was £20.
Because my dad does get confused sometimes, I wasn’t sure if there was a misunderstanding. So I asked in the group chat whether I was supposed to pay £10 or £20. I explained that I couldn’t really afford £20, but if my dad was lending it to me then I’d be okay, I just wanted to know which amount was correct.
My brother then replied saying he was cancelling the meal, that organising anything with the family is always difficult, and that he felt like I was accusing him of trying to rip me off. That genuinely wasn’t my intention. I was just confused about the amount.
I told my mum about it, and she thinks I’m the asshole, that I sounded ungrateful and should have just paid the £20.
But I honestly wasn’t sure whether my dad was mistaken or whether it really was £20.
If it had been £20, I was willing to pay it. I just wanted confirmation as I was just confused that’s all.
But now I feel bad like I have thrown his kindness back in his face but also feel like it was a perfectly acceptable question to ask!

So, am I the asshole?