Not a common idiom. It is probably meant to be criminal/pickpocket slang. It may have been made up by the writer to sound like slang, with no basis at all in "natural" English.
From the context we can guess that "leather" is a slang way of saying "wallet" and "flip a leather" is supposed to mean "steal a wallet". I've never heard this expression before, but given the context it seems to make sense.
Now I guess that it is easier to pickpocket from someone who is standing up (because their attention is elsewhere). So probably it means "steal a wallet from someone who is standing up". The suggestion is "I'm sure you can do this easy type of stealing. (But I'm asking if you can do something much harder)
It is quite common for movie writers to invent slang. Lots of the "gangster" slang in movies like Goodfellows and Godfather was invented by the writers (and later picked up by the actual gangsters, who wanted to sound like the guys in the movies)