The potential deal, and any deal Comcast and Charter might agree to, is entirely contingent on the success of Charter's proposed acquisition of Time Warner Cable, sources say.
And while Comcast and Charter are close to agreeing to terms, Charter and Time Warner Cable are still far from a deal, sources tell CNBC.
Both Comcast and Charter declined to comment.
A lot has to happen before Charter would be able to sell Comcast those cable assets. So why are they negotiating now?
Because it would help Charter understand what it should pay for Time Warner Cable, based on how much it could get for those plum media markets. And of course this big sale would help Charter raise cash after borrowing to finance a Time Warner Cable acquisition.
This game of cable industry cat and mouse comes as Liberty Media CEO John Malone pushes for industry consolidation. Charter, which his Liberty Media owns 27 percent of, made a $38 billion offer for Time Warner Cable, which it swiftly rejected.
And it makes sense that Comcast is interested—though it operates in 39 states, this kind of deal would give it access to entirely new, valuable, areas, like New York City.
Disclosure: Comcast is the owner of NBCUniversal, the parent company of CNBC and CNBC.com.
—By CNBC's Julia Boorstin. Follow her on Twitter: @JBoorstin.