About to face a valuable employee to reduce his pay by 25%. Due to unauthorized raise given by previous manager. Any thoughts to make it easier for him to accept?

The previous manager before leaving gave the most valuable employee a pay raise by 30%. Which makes him the highest paid worker of this profession in the industry. I guess it was a way to stick it to the company. Now I have to do the reversal, and there is a chance we are going to loose him. I am not sure if there is anything else to do other then just tell him and explain the situation and hope he understands?

Edit: correction. The raise was almost 50% of his pay., now to roll it back it would take about 30% cut.

Edit: he quit. Taking day off, will pick up his stuff tomorrow.