Should I try to keep the rasp?

Of course!  Kleptomania is the first skill of all good adventurers.  But
you'll have to get Max to give it to YOU instead of back to the soda jerk.

First, you'll need some privacy.

The soda jerk might not want you to keep the rasp, so you'll need to go
behind his back.

Or, rather, out his door.

Now, if you can just get Max's attention.