Tenant who may break lease early

Hello,

First time post here, and was hoping to get some wisdom/guidance. I've been renting out my townhouse through a property manager for the past 2 years, and have absolutely loved the tenant who has lived there. 

The lease renewal letter was sent out on May 30th. The deadline to sign was June 30th. In the lease, it states that if it is not signed, they accept the terms of the new lease. Seems like pretty standard language.  Long story short, the tenant did not sign, and is now indicating they may not want to stay for the full term as they are interested in purchasing a home in the coming months.  She asked if I would be willing to negotiate to a shorter term or even slightly lower rent amount (was increased from $1,875 to $1,960--which is in line with market value for the area).

My main question is this: as a landlord in this scenario, what legal grounds do I have to stand firm if she states that she wants to ultimately leave and break the current lease? Would she be responsible for the rent until I find a new tenant? That is what I've been reading. 

Also in this area. If I pay for property management to procure a new tenant, could I charge her those fees?  I feel I have been a responsive landlord and even just replaced the HVAC unit 1 month ago. The last thing I want is to have a vacancy, especially when the lease has technically been renewed. I am all for being fair, but also don't want to be taken advantage of. Any advice or knowledge that could help would be greatly appreciated!