The 27-year-old Cameroon international spent last season on loan with the Hammers and impressed sufficiently to earn a permanent move and a three-year contract, as long as the former Arsenal midfielder can prove his fitness.
Speaking to the KUMB (Knees Up Mother Brown) Podcast, Sullivan said: "We have a deal in place with Alex Song subject to a medical. If the medical is okay, we'll take him.
"He couldn't physically pass a medical tomorrow, but if the medics say it's just a technical thing and he's only a couple of weeks away you'd take that gamble and hope they're right.
"He's still got two years left on his Barcelona contract and you're signing a player on a three-year contract, so it's a huge commitment. He's a big earner, Alex.
"He's injured at the moment, which is something you've got to weight up. But we've got until September 1st to make a decision."
Song starred for West Ham in the first part of last season but his form faded like the team's.