The net profit of the guest is equal to Xiao Zhang's net loss, so Xiao Zhang loses 40 yuan.
It's a gimmick to exchange counterfeit money with your neighbors. Don't think about it. Because the neighbor gave Xiao Zhang 50 yuan real money, and then Xiao Zhang took out real money from 50 yuan and returned it.
Two don't owe each other.
That is to say, the guest exchanged a counterfeit banknote from 50 yuan (zero cost) for 20 yuan's shoes, and exchanged a real banknote for 20 yuan's, and Xiao Zhang received one, but none.
With counterfeit money, I lost a pair of shoes and real money from 20 yuan.