This is a personal matter, my dear.
3 seconds sound clip from the Monsieur Verdoux movie soundboard.
You can hear this line at 00:48:01 in the Blu-ray version of the movie.
Quote context
[...]
- Louis, this is Monsieur Challain. This is my husband, Captain Bonheur.
- How do you do? I am president of the Salt Water Fuel Company.
- Here's 5,000 on account. I'll send you the rest tomorrow.
- This is a personal matter, my dear.
- Something that might interest you, monsieur. It's a device for turning saltwater into gasoline.
- How dare you, sir! Don't you do that! Don't you...!
- Don't you ever dare humiliate me again in public.
- I merely tried to stop you from making a fool of yourself.
[...]