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.
The actual quote by Lando is:
I had no choice. They arrived right before you did. I'm sorry.
taken from comment by nicolallias