Who do I make the check out to? The law firm or their client that they are representing?

My husband had an old Chase account that got turned over to a law firm. We are mailing a money order to the law office to pay the balance in full. But who do I make the money order out to? The law firm or Chase? I can’t get a hold of the collector we’ve been in contact with to ask him, so i come here 🙂