In my mind I can remember Han Solo or Lando saying "he made us an offer we couldn't refuse" in reference to Darth Vader. Did I imagine it?
Here's the script for The Empire Strikes Back and there's no quote that's same or similar to what you have mentioned.
You probably did imagine it.
Edit: The actual quote by Lando is
I had no choice. They arrived right before you did. I'm sorry.
taken from comment by nicolallias