Agreeing with tenant to leave before lease ends

[John Collins]

I am an owner of a rental property where the lease ends in 4 months and know the tenant would like to move out before (moving into home with fiancee), but is fine staying til the end. This works out well for me as the property has always had a low ROI , will never increase in value and might decrease with newer developments planned.

Plan to sell the place on my own (at a low , attractive price point) - to use the cash to invest in a flipper I have already identified. 

How can him leaving before the lease ends legally work - does a contract have to be drawn up, or can he just move out at the end of June/July with me cleaning and listing the property? I wanted to know the exact legal protocol for Harris County, Texas. Don't want to run into a situation where I list it for sale and the sale doesn't go through because there's a contract out there that says he is leasing it for a few more months. 

Thank you, all help is appreciated.