The times I've deviated, I've let them buy the scheduled ticket first. Then I've deviated and cancelled that flight once I have an email of the actual fare they paid to get me out of MEM.
When the fare (on the scheduled DH) that they bought has been higher than the accepted, the actual (higher) fare on the purchased ticket I cancelled has gone into my deviation bank.
Usually the accepted and actual match, but when the actual has been higher, they've given it to me in my bank (as long as I let them buy the ticket first). That's probably not the way it's supposed to happen but it's been consistently done that way for me.