When costing an ad hoc charter, empty legs are costed into the price.
Re selling the empty legs, it depends where it is going to. Generally you would sell empty legs cheaper than normal as they have already been paid for. It would be up to the empty leg buyer whether they took it one way or return
Re cancellation by the principal, it depends on the contract, how much is due when they cancel. Airlines will not run ad hoc charters unless they are making money on it. If the principal cancels and there is not enough money the whole thing will be cancelled. It would commercial suicide to run ad hoc charters at a loss. If the empty leg buyer could be persuaded to pay the price quoted to the original principal, the the flight would operate... then there would be more empty legs to sell!
FYi not alot of empty legs are sold - there is also a few websites for the sale of empty legs - just google it
hope this helps
SR