When Hermione began asking about the engagement, he didn’t want to bring up the breakup. He was hopeful that he’d gotten out of it, but since Astoria was still at the Manor and he had only recently had that conversation with his mother, he needed to be sure before he truly announced it. He wasn’t lying when he said they had an agreement; of course, Astoria was the only one actually taking advantage of it. The only person Draco wanted had been unavailable up until  right now. 