At the end of the story, Oliver discovers that the pearls that the Duchess has given him are worthless. They are not real and so that means he has paid her a huge amount of money for fake jewels.
Oliver has done this because he is so obsessed with rising above his humble childhood. He wants very badly to mix with high class people like the Duchess. She invites him to come stay at her estate and so he does not really want to jeopardize that invitation by looking too closely at the supposed pearls.
The Duchess and The Jeweller is a short story by Virginia Woolf. Oliver is certainly a gentleman who is something of a perfectionist with the "proper allowance" of everything he needs. He is a creature of habit and basks in his own successes. He suggests that he has proven his worth to his mother, presumably deceased, whose painting hangs on the wall, and now he is "the richest jeweler in England." The reader cannot help but sense the pretentiousness of Oliver and he is certainly an insincere character, "a sad man, a dissatisfied man, a man who seeks something that is hidden." His business is so successful that it is well-known universally and people even "envy" him but still his life lacks something. He recalls his childhood and how he "began life in a filthy little alley"; he is painfully aware of his humble beginnings. This causes insecurity in him and his constant need to reassure himself highlights his low self-esteem.
It is understandable, therefore, that Oliver lives a conflicted life and he and the Duchess, who is herself a character who cannot find peace in her life, "each cheated the other, each needed the other." Their relationship is counter-productive for both of them. He knows from the start that there is a possibility that the pearls she is offering him could be fake but he does not have the correct lighting to make a reasonable estimation of the pearls' value and he must weigh the risk of exposing her lie. As a lonely man whose wealth has not brought him happiness, Oliver is therefore willing to trade some worthless jewels just for an opportunity to mix in the correct circles. The value of the pearls - which are obviously fake - becomes insignificant but still real value and true friendship elude him.