No healing is wasted.

Encounter ✦     Divine
Free Action      Close burst 5

Channel Divinity: You can use only one channel divinity power per encounter

Trigger: A healing power restores an ally within 5 squares of you to his or her maximum hit points

Target: The ally targeted by the triggering power

Effect: The target gains temporary hit points equal to any hit points the triggering power restored in excess of the target's maximum number of hit points.

Published in Eberron Player's Guide, page(s) 90.