I don't think I'd be mad at my best friend at all, really. After all, when the offending incident happened, they were five, too and far too young to realize that this could be painful for me. I'd probably wonder why they didn't tell me, but that's all.
I mean really, all it would've taken was a single "Hey, that one time, when we were five, your mom gave me ten bucks to hang out with you."
I think I would be a bit annoyed by my mom, but I'd ask her what her reasoning behind all that was before jumping to conclusions.