Circumstance makes Harry an offer he can't refuse.
“I can get you the Elixir of Life,” Harry says. “But I need your help with something else.”
“You,” says Voldemort, cautious now, wary now that this confrontation hasn’t gone at all like he expects, a little incredulous, “need Lord Voldemort to help you?”
Harry looks right past him for a second, jaw tense. “Yeah. You’re the only one who can, apparently.”