If Maria the caster casts charm person on Bob the merchant, Bob will have no immediate reason to be suspicious of Maria--the spell ensures that Bob views Maria as a decent enough person. If Maria tells Bob to give her something for free, then unless Bob is the type of merchant who normally gives or lends out stuff for free in the first place.
If Maria swears up and down that she'll give it back, she just needs it for a moment, and then makes a good Deception roll (made with advantage!), then there's a pretty good chance that Bob will believe her. By which I mean, he will believe that Maria is being honest here, that she intends to bring back the thing. This assumes that Bob doesn't know Maria or has reason to believe she's a liar (assuming she is lying here).