@Sandy109
As I see it:
They got a monthly discount, thus bringing down the single night price lower than if they hadn't booked a month's stay.
If cancelled days bring the length of stay down to less than a month, they would then be paying 28 x your normal nightly rate.
Are you sure that with their cancellation it isn't YOU who should be getting some extra payment.
I mean, if I sell potatoes at a wholesale price from 30 bags on, and at a retail price under that,
then if you buy 30 bags at my reduced price, but gave back 2 bags, you'd not be getting money back from me.
Honey, you'd have to pay me back to cover the difference in wholesale to retail price for the remaining 28 bags!
(Sorry, just realized I'm repeating @Deborah197's comment in other words, but will leave it anyway...)