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