When viewed in a thrift store stereo

When viewed in a thrift shop of a stereo tape recorder, a customer accidentally damaged it. The commissioner discovered the damage only a few days later. Since the tape recorder could not be sold at a price agreed upon with the consignor due to damage, the store lowered the price and sold the tape recorder twice as cheap as was provided for in the contract. The committent demanded the payment of the entire amount of the cost of the tape recorder, determined by the contract, and also stated that the commission agent did not have the right to claim to receive a commission, since he had performed his duties improperly. The commission store paid the cost of the tape recorder to the client at the price of its actual sale and withheld a commission.
The principal went to court.
Solve the case.