Summary
I was a little surprised that Harkless only got a one-year, $3.6 million contract last summer. While there's a possibility he turned down more money to join a competitive team in a glamor market, it's doubtful he turned down anything too much bigger. The Heat signed Harkless in the hopes that he could help fill the role that Jae Crowder left behind when he signed with Phoenix last offseason.

Unfortunately for Harkless and the Heat, Harkless was unable to contribute in any meaningful way, and he was traded to Sacramento. He was never expected to be the shot-making power forward that Crowder was in last year's playoffs (it also was probably unrealistic that Crowder himself would be continue to hit shots at that rate, which is one reason, along with cap space considerations, that the Heat let him walk away in free agency), but he was expected to be a versatile forward that would hit enough outside shots to be respectable. Harkless did have some nice moments for the Kings, but likely didn't do enough to improve his market going into free agency.
 
Cap Considerations
Harkless will be an unrestricted free agent with a $4.3 million cap hold and Non-Bird Rights. Using his Non-Bird rights, the most the Kings could offer him is a starting salary of $4.3 million, which would likely be enough to retain him. If necessary, they could also use cap space to re-sign Harkless, or the MLE if they remain over the cap.

Other teams will have multiple ways to sign Harkless as even teams without cap space could possibly use either of the Non-Taxpayer MLE, Taxpayer MLE, or possibly even the Room MLE or BAE to sign him. There's also a chance that Harkless could be had for the minimum.