Master Wang sells shoes with assets of 30 (the value of shoes), liabilities of 0 and owners' equity of 30.

The money given by the customer is assets 80, liabilities 50 (shoes and change owed to the customer) and owner's equity 30.

There was no change when I exchanged 50 yuan with my neighbor. Assets 80 liabilities 50 owners' equity 30.

Give customers 30 yuan's shoes, and give money to 30 yuan when exchanging money. At this time, Master Wang spent 30 on inventory (shoes), 30 on cash, 20 on assets, 0 on liabilities and 20 on owners' equity.

The neighbor asked Master Wang to compensate 50 yuan. At this time, Master Wang's assets are -30, which is worse than 30 yuan at the beginning.

To sum up, Master Wang lost 50 yuan.