Your story is confusing. What exactly do you think is wholly unfair? From what you've written, all I gather is that you've been paying rent $600 under market, you needed to terminate your lease early, your landlord agreed to let you terminate on one month's notice, and they're trying to find a new tenant by February 1 so you don't even have to pay that one month. What part of this is unfair?

If you and your landlord agreed to terminate your lease at the end of February (i.e. no matter what happens, you must move out by the end of February and February is the last month for which you owe rent), then come March 1, you are history, and the landlord is free to re-rent the unit to a new tenant at any price. Moreover, why do you care what rent a new tenant might be paying?