Chick is right. It does not get simpler than that.
If Lee had a contract that was signed buy him and Chang for the last event, and in it it included air fare and Lee did not go, but used the airline tickets for personal use, then he OWES Chang the difference....on that contract and whatever else was not completed per the contract.
Assuming there is another new contract, and ASSUMING that Chang and Lee both agreed and signed it, and it included airfare, then Lee is bound to use the airfare to get there for this event and do whatever the contract stipulates.
What is not known is if there IS a contract with that provision for this event, and if it was agreed to and signed by both. If not, Chang can not pay whatever he wants since Lee broke the first contract first by not going and then, cheesy as it is, used the tickets for his own personal use.
Since it seems many pros seem to have a mind block to even the slighest concept of a legal document, it's my take that Lee probably told Chang that he used the tickets, and Chang now is just getting back what is owed him, no contract involved. What is wrong with that? He used the tickets to fly somewhere else, and WTF Lee can't swing a trip to the Olympia on his own dime? That's fucking sketchy.