It may seem to many readers that Mathilde Loisel, or her husband, or both of them should have gone to Madame Forestier immediately and explained that the borrowed necklace had been lost the night of the ball. They could have offered to repay her for it in installments. If they had done this, she would have told them it was only worth about five hundred francs, which would have been equivalent to about a hundred dollars in those days.
Maupassant does not explain why the Loisels do not even consider doing this. Once they realize the necklace is lost and may never be found, Monsieur tells his wife:
"You must write to your friend," said he, "that you have broken the clasp of her necklace and that you are having it mended. That will give us time to turn round."
The end up borrowing a small fortune to buy a duplicate necklace and then spend the next ten years paying back all the loans and the accumulating interest. There are three possible reasons why they do not tell Madame Forestier the plain, honest truth and offer to pay her for the supposedly highly valuable lost necklace in comfortable installments, in which case they would have found out, to their unspeakable relief, that they only owed her around five hundred francs-- which would have been a bad scare but a good lesson.
In the first place, they both might have been afraid that Madame Forestier would not believe them but would have thought they were making up a story in order to keep her diamond necklace and probably sell it at some time in the future for a profit of around thirty-six thousand francs. They may have been afraid that Madame Forestier would refuse their offer to pay for it in installments because she wouldn't trust them. If she thought they had stolen her necklace, she certainly would trust them to pay her for it.
Even if Madame Forestier believed their story and accepted their offer to pay for her necklace, her husband might not believe them and might not agree to such a repayment plan. After all, it must have been the husband who originally bought the necklace, and he might be a lot harder to deal with than his wife. If he accepted their offer, he might have to go out and buy his wife another diamond necklace for forty-thousand dollars. Instead, he might go so far as to report them to the police! He could not prove that their story was not true, but he could create a scandal.
Finally, a scandal is probably exactly what Monsieur is concerned about. He has a sensitive job with the government. One can imagine the stern, demanding superiors he works for! If the police started making inquiries about him at the Ministry of Public Instruction, it could create trouble for him and affect his future career. The loss of the necklace could even be reported in the newspapers, and everybody in Paris would know about. As in Maupassant's story "A Piece of String," there would be some people who would believe the Loisels' story and others who would suspect them of being jewel thieves!
It is Monsieur Loisel who takes the lead in deceiving Madame Forestier and in borrowing all the necessary money to buy a genuine diamond necklace to replace the one his wife lost. His motive is probably the dominant one. Madame Loisel would have no way of raising anything like 36,000 francs. By herself she would have no choice but to go to her friend in tears and confess that she had lost her necklace. Ironically, this is exactly what Mathilde does at the end of the story, but in the interim she has lost her beauty, her figure, her charm, her priceless youth, and her dreams of glory.