I’ve wondered the same thing, why not wait until she’s had her dance with Lord Debling and then ask her to dance so they may finish their conversation?! Unless it was getting towards the end of the ball with no opportunity to talk afterwards. He could’ve also bribed Rae again so he could talk to her without outside interruptions.

I thought it was because he was scared debling would ask for her hand during the dance. He wanted to take any time she had with debling alone.