Don't see this as a scam.

Three things can happen here:
1) Server bugged and now he had additional troops appearing. (I've seen people report both that they lost troops and they have gained them.)
2) He was waiting on reinforcements from his alliance members, which could be multiple people adding the troop size to 40k.
3) They used camp time to Punk you. They waited until the armies arrived right before your army did and after you had scouted.