Pricing alteration leads to unexpected price increase

[Adam119]

In two instances, one recent and one upcoming, I've sent alteration requests to pricing and heard back from the guest that the amount they were being asked to approve was different than what we'd agreed upon.

In one case, I altered the subtotal of the stay from $155 to $175. The guest's total for the original was $192 (including service fee and occupancy taxes), but when I added the alteration, their total became $217 (an increase of $25, instead of $20).

In another, I've attempted to discount a stay's subtotal from $350 to $335 (a decrease of $15), but the guest (a first-time Airbnb user) was confused: "It says if I accept the 18 dollars back... I would charge me 22 dollars which doesn't make sense"

Is this behavior expected? How can I alter prices to give guests a specific amount off or charge a specific fee?