How can I remove a potent smell from leather?
Cigarette stench is a bad one - good luck, you'll need it. I would have moved on to a different sale, personally. 

You could try using saddle soap (buy it at a tack shop) and scrubbing the surface down. Follow the instructions on the can. Might need to repeat 3-4 times.

If there's a fabric bottom, remove, discard, and replace it.

You might also consider replacing the stuffing, if its permeated deeply.