AITA for not paying my sister back for the weed I bought from her, and for arguing with my fiancé about it?

Here's the situation: I'm in my late 20s/early 30s, have two kids, and am engaged. I've recently moved out of the house to live with my fiancé and his mom, but ended up going to a shelter because I couldn't stand living with his mom anymore. My fiancé didn't come with me.

For a couple of weeks, I bought weed from my sister. Each time, I bought four 8ths. In my area, weed typically costs $15-20 per 8th. My sister says I owe her $300, but my fiancé claims he paid her $200 for the weed and gave her an extra $100 because she needed it.

Am I the asshole for disputing this and not paying her the $300 she says I owe?

What was owed was less than 100 and he just gave her $300 for nothing talking about that his sister etc etc when they are constantly asking him coming to him for money even though they have boyfriends. He’s the only brother/son in the family.