@Yvonne953
First....I would be cautious regarding a guest who asks for a 2-night stay when your minimum is 3nights. What are their reviews like? The guest will likely already be disgruntled from the get go if you charge a premium for a 2night stay and be looking for reasons to complain and/or leave a negative review because they were charged a higher price for a 2night stay. I have seen this situation and it has never turned out well for the Hosts I work for. It is like asking for a discount and those guests are usually a headache and you risk a negative review from the guest even if they say they are fine with the higher price.
That said, there are several ways to do this if you want to allow a 2night stay. Any way you do it, you'll have to explain the nightly rate will be higher to the guest. Either because you raise the nightly rate, or increase the cleaning fee. One way is to send a Special Offer and be sure to add a higher cleaning + the nightly rate in the total.
Sending Special Offers
https://www.airbnb.com/help/article/35
My instincts tell me this might not be a suitable guest for you and your property and could be more headache than it's worth, but that is up to you of course.