@Taylor454
It would be a good idea if you familiarise yourself with Airbnb's cancellation policies, especially the long term one, which applies to all stays of over 28 days, so this sort of mess doesn't happen in future. This is nothing new. I have been hosting long term guests for years and it is completely normal and spelt out in Airbnb's policies. For long term stays you get paid monthly, regardless of how much money Airbnbtakes from the guest up front.
So, if it has been the case for years that, if a guest books for a month or longer, you get paid the first month after the guest checks in and then in monthly instalments thereafter. Therefore, the payment was correct. You were due the remainder one month after after the guest checked in.
According to the long term cancellation policy, the guest must pay the first month if they cancel before the stay. After they check in, they must pay for the nights they have already stayed + one month from the date they cancelled. Therefore your guest owed you for the nine days she stayed + one month. The rest would have been automatically refunded to her by Airbnb anyway. You didn't really need to do anything other than let her cancel and pay what she was supposed to. You would have received what you were promised (according to the cancellation policy on the booking) even if you did not get it all straight away.
Now, if you agree to let a long term guest check out earlier and not charge them for according to your policy, then you could instead simply alter the check out date to whatever you have agreed with the guest. You just send an alteration request and the guest accepts it. She then gets refunded accordingly and you get to keep the portion of the accommodation fees that you agreed to.
By not knowing the policies you agreed to (like I say, they haven't suddenly changed), you've put this into the guest's and Airbnb's hands and ended up with an Airbnb rep who has made an error maybe due to inexperience or maybe because the situation was confusing and unclear.
You can get this resolved, but first you need to make sure you know what you are talking about regarding Airbnb cancellation and payment policies. I hope you get it sorted.
PS There is a weird long, long gap after your post, which is unusual. Kind of like a formatting error. This may prevent others from responding to you because you have to scroll and scroll before you get to the option to respond. I have never seen this on other posts, so maybe edit yours to make sure you have not included a huge blank space under your text.