If you should happen to change your mind before he leaves...
3 seconds sound clip from the Criss Cross (1949) movie soundboard.
You can hear this line at 00:08:08.421 in the DVD version of the movie.
Quote context
[...]
- You didn't happen to see anything, did you, Walt?
- Neither did you, Vincent, huh?
- Can I go out now and wash up? Is that okay with you, Lieutenant?
- Go and wash. Go to... Detroit.
- If you should happen to change your mind before he leaves...
- I'll let you know, Lieutenant.
- Chump.
- What went wrong? Why did you have to go and pull a knife, Slim?
- He flared up. In the middle of everything, he started to ask a bunch of screwball questions.
[...]