Apparently these people never heard that you're not supposed to mix business with pleasure, a.k.a. don't lend money to your friends. So much money is made, inherited, borrowed, paid back, not paid back, and lost in Sentimental Education that it can be hard to keep track. All you need to know when it comes to money in the novel is this: everyone wants it.
Monsieur Dambreuse is the only person in the novel who has really earned their wealth.
Frederick is horrified by Madame Dambreuse's behavior at the auction of the Arnoux's possessions, but he's also disappointed that she didn't inherit all of her husband's money.