@Gayle29
I would think the simplest way would be to have the guest change the dates to include 1 more night per the current rule and have the guest send a change request. (This kind of thing should NOT be done by the host. If the guest wants to extend their stay, they need to make the change request, not the host.)
Once you recieve the change request, you (host) can "Send special offer" and reduce the amount by 160 dollars. Because of Airbnb host/guest fees, the difference between the original and final payment amount to be made by the guest may not be 160 dollars. Sames goes for your payout.
Happy hosting~!