Spike doesn't care about money as much as justice. He saw that woman kill their bounty and a bunch of innocent by-standers. He was well aware of how nervous she was getting as he tried to get that vial out of the outer wrapping. So he "gifted" it back to her, figuring it would be her just desserts.
When I first watched Firefly I expected some influence just from the similar genre mashup, but then they found a mysterious frozen lady in the luggage and it became clear how much they were inspired.
Re: why did he slip her the vial? I assume because he noticed her reaction to him trying to break it open and figured out what it was... or at least that it was dangerous.
Spike is a very observant guy. Don't be fooled, any indication to the contrary is an act. You think he'd not notice the woman who was calm cool and cocky about her status as a prisoner not starting to sweat over his goofing around with a little knick-knack and not think something was up? Of course you'd think that he wasn't even looking or paying attention to her while he was doing that but, the man is a legend for good reason.
