Han and Lando have a conversation about it for starters
HAN: Aren't you afraid the Empire's going to find out about this little operation and shut you down?
LANDO: That's always been a danger looming like a shadow over everything we've built here. But things have developed that will ensure security. I've just made a deal that will keep the Empire out of here forever.
Later - Lando says to Vader:
LANDO: You said they'd be left in the city under my supervision.
As well as this:
LANDO: Lord Vader, what about Leia and the Wookiee?
VADER: They must never again leave this city.
LANDO: That was never a condition of our agreement, nor was giving Han to this bounty hunter!
VADER: Perhaps you think you're being treated unfairly.
VADER: Good. It would be unfortunate if I had to leave a garrison here.
LANDO: He doesn't want you at all. He's after somebody called Skywalker.
LANDO: Lord Vader has set a trap for him.
Given the context, it seems apparent that the deal is that Lando helps Vader obtain Skywalker, in which case the Empire will turn a blind eye to Lando and Cloud City, which, given his initial conversation with Han, is apparently in danger from getting shut down otherwise. Whether or not Lando is to be trusted in these comments is up for debate.
"The Empire Strikes Back", The Internet Movie Script Database