Oliver Bacon the Jeweller most definitely experiences "loss" in a material sense in the transaction with the Duchess. Specifically, he loses the twenty thousand pounds that he pays to the Duchess in exchange for fake pearls. To confront her about it might mean ruining his chances of having a romance with her daughter Diana. There is also a secondary loss in terms of Oliver's self-respect. His instinct is that the Duchess might be planning to swindle him with fake jewels, but temptation overcomes Oliver's willpower as he thinks about the prospect of gaining access to Diana. Even as he writes the check for twenty thousand pounds, he hesitates. After giving her the check, Oliver discovers that the ten pearls are indeed fake. He then looks up at the painting of his mother and asks for her forgiveness. His mother would not have approved of this imprudent purchase without proof that the pearls were genuine.
On the other hand, Oliver is the comparative winner of the psychological upper hand in the transaction. He knows that the Duchess is in a desperate situation because of her gambling problem. He also knows that her husband would take drastic actions against the Duchess if he found out that she had been gambling away their wealth and pawning family valuables. To uphold her so-called "honour", the Duchess needs to cover up for her gambling losses. Oliver could have refused the sale, leaving her to suffer the consequences with her husband. Instead he makes her even more obligated to him by providing her with the money. As the keeper of her darkest secret and the victim of her crime, Oliver has the power to destroy this woman's reputation if he should choose to expose her. The Duchess would do well to appease him by letting him court her daughter whom Oliver loves.