When he runs into the pimp later in the movie, the pimp
does
use the F word. At the time the unwritten rule for a PG-13 rating was that multiple instances of the F word would bump the rating to an R. So I wonder if the first overdub wasn't to ensure a PG-13 rating and thus a wider audience? Script-wise it would have made more sense to drop an F bomb in the first scene and do the overdub in the second. Not sure why they did it the way they did.

the strong language warranted him punching the guy, but "screw" isn't that offensive.
Tell that to Rosie Perez in White Men Can't Jump.
There are maybe five beings I hope I never have to fight and J'onn Jonzz is one of them. - Superman

It's such a shame, the strong language warranted him punching the guy, but "screw" isn't that offensive.

You are not mistaken. When I first saw this film in theaters back in 1986, the F-word was used but once it was edited for TV it's obvious that the pimp doesn't say that, even tho his mouth looks otherwise.