Actually, it's not all that easy to find the correct meaning in a dictionary (although the comments under the question probably should have helped quite a bit). Looking at Merriam Webster we find for short, under verb:
1 : short-circuit
2 : shortchange, cheat
3 : to sell (a security) short in expectation of a fall in prices
option 1 is about electricity, and not relevant in this case. The third option is about the stock-exchange, and not relevant either, although that may not be clear immediately.
The second option is what we are looking for. We can actually substitute cheat for short in the given dialogue without really changing its meaning.
If we look up shortchange we get:
1 : to give less than the correct amount of change to
2 : to deprive of or give less than something due
Where the second option is a more detailed explanaition of cheat.