If you know your daughter well, and that she wouldn't behave in a bullying way, then my instinct would be to let it be. I'm willing to bet that this little girl will be asking to play with your daughter again sooner or later, and may well have to tell her mother that it wasn't quite how she told it.
If you want to address it, I would ask the mother for more information. You could take the role of concerned parent, like you are wanting to make sure your daughter doesn't behave in a bullying way towards younger children, and ask her to be specific about what happened. That way, you can get a clearer picture of what she classes as bullying, and the mother may well need to probe a little deeper and find that her daughter has just "fallen out" with yours, and its just a case of kids being kids.
It is upsetting when someone labels your kids in a negative way, but, this woman is really just penalising her own child, because she is denying her a friend.
The other side of the coin is, perhaps the mother is being protective, and doesnt realise her daughter was being a little manipulative. She might feel that, since her daughter is 2 years younger, then yours must be at fault. Its understandable that she would take this position, but an unfair and extreme reaction to call your child a bully.
If you know her well, by all means try to explain your side. If not, you may risk coming across as defensive, and it may not be well received.
I hope this helps a bit, I know I haven't given much practical advice, I just thought I would give you my take on it, just to get an outsiders point of view.
If you do decide to talk to the mother bear in mind that like you, she will want to believe her daughter so you may get no further than just putting your point of view accross. Doesn't sound like bullying, sounds more like your daughter just resented having to play with someone she'd already said no to and the other girl got the hump when she couldn't get them to change games for her. Next time, let your daughter be the one who decides who's company she does and doesn't want.
If you think about it, if you'd made plans with a freind then another one interupted them you'd resent them being there and theres no reason why it should be different for a child. I've done exactly as you did several times and the result is generally the same.
