our property management agreed to "work" with us to terminate lease early,now advertised rent much higher than market will get, is it legal?

The issue here is that your landlord is holding you to your lease until he or she finds a new tenant and you do not believe that he will find a new tenant because he is demanding above market rent.   This is not illegal.  But if you break your lease he may not be able to collect forward rents from you for the remainder of your unexpired rent if he is advertising the unit at a much higher rate.  Thus, not making a reasonable attempt to mitigate damages.   Good Luck! Don Eby