Let me guess: Rufus is getting a lot of offers for marriage, and he has zero interest in them. They keep disturbing his studying and training. If he already has a fiancée, it's the best possible excuse to reject any further offers immediately. Furthermore, Rosalie isn't actually a totally random person, she's still a neighbouring count's daughter, so while everybody knows she's poor, if Rufus is not wanting for money, her background would still appear to be of believable interest for him.