This request you have received from a guest, does it apply to a split payment maybe payable in two or three instalments? There is no advantage for a guest to pay ahead so am perplexed why they are keen to pay ahead? They could I suppose contact Airbnb to see if this is possible as Airbnb handles all the payment side of the business, not us.
But do let us know if the guest is able to change the way they pay the reservation