Well, unless you are playing in some type of Orgainzed Play (like Living Greyhawk) then there isn't any big deal. The XP rules are more like "guidelines" anyway. 
Fiarness is all in how you and your players agree on something. Letting them keep their levels is just a good idea since they have already leveld up and it is annoying to "back-out" a level. Having them "owe" the XP is fine. Just let the players know what is going on and that they hit 8th level a little soon.
-The Luddite

Fiarness is all in how you and your players agree on something. Letting them keep their levels is just a good idea since they have already leveld up and it is annoying to "back-out" a level. Having them "owe" the XP is fine. Just let the players know what is going on and that they hit 8th level a little soon.
-The Luddite