@Laure3919 Refunds are based upon the total cost of the booking, not on how much you paid upfront. That is stated in the cancellation policy info if you really read through the terms of the cancellation policy. A lot of guests are confused about this, so I guess Airbnb should make it clearer somehow to guests.
The pay less upfront option is just a convenience to guests so they don't have to come up with the entire cost of a booking at once, but it is unrelated to the cancellation policy.
If you really think it through, you'll understand why it makes sense. Let's say that we both book the same place, for $100/night for 4 nights, but you chose to only pay half up front and I chose to pay the entire booking.
We each need to cancel and are due back 50%. But I paid the whole shot up front, $400, so I get back $200.
You only paid $200 to start with, so you get nothing back.
We each forfeited $200.
There's no logical reason why the guest who opted to pay in two installments should only be out $100, when I am out $200- that would be totally unfair.