The best way forward would be to explain to the buyer that leaving an item to be posted at a later date would incur you a late delivery mark on your account, so best to cancel the order, and relist when you are back.
With their agreement cancel citing ' Buyer Requests to Cancel, this fully refunds them and your seller performance is not affected.
Then, relist when you're back for them to buy, and pay again.
@rowil-3225