The great painter, Johannes Vermeer, passes away toward the end of the novel. His death becomes the catalyst for an important decision. His wife, Catharina, has been instructed (through the stipulations in her husband's will) to bequeath the pearl earrings to Griet. While Catharina isn't too happy about handing the earrings over to Griet, Griet herself is just as unhappy to accept such an unfortunate gift. The pearl earrings have been a source of jealous conflict between Catharina and Griet since it was discovered that Griet posed for a portrait for Vermeer while wearing Catharina's prized earrings.
The discovery prompts Catharina's violent attempt to destroy the painting; Griet leaves her master's employ because such a conflict would inevitably render her place in the household insupportable. Griet decides that she will marry Pieter after all. After her marriage, she helps at the family butcher stall. Even though she sometimes sees Tanneke (the Vermeer housekeeper) at the market, Tanneke now buys her meat from other butchers.
As soon as I began working alongside Pieter they had switched butchers—so abruptly that they did not even pay their bill. They still owed us fifteen guilders. Pieter never asked them for it. “It’s the price I have paid for you,” he sometimes teased. "Now I know what a maid is worth."
The matter of these fifteen guilders is brought up again at the very end of the novel. Griet sells the pearl earrings for twenty guilders at a pawn shop. She reasons that keeping them would mean more trouble than it's worth. After all, how could she admit to her husband the manner in which she came by them? She tells us that a butcher's wife does not wear such expensive ornaments, nor do maids. The implication is that if a maid did wear such earrings, local gossip might insinuate some impropriety in her relationship with her master.
Griet tells us that she now has five extra guilders she would not be able to explain to Pieter. She intends to hide them so that Pieter and her sons will never find them. Fifteen guilders would go toward the debt that the Vermeer household owes Pieter for previous orders of meat. The five extra guilders will be her secret memento; they are the only link to her memories of her brief infatuation with her master. With the fifteen guilders, Griet will also be able to put to rest Pieter's joking claim that a maid costs a certain amount of money.
Hope this explains things for you!