There is this software engineer in the team I lead who in the past was a very good worker and in the last two rounds got a salary increase of X%, the maximum the company allows. The last time, a couple of months ago, he said he was very disappointed and this raise doesn't reflect his hard work and the stress he's under, so he'd "keep his productivity increase to X%".

And true to his word, he has slowed down his work pace. He still does things well, even his attitude towards the rest of team has improved from his previous light grumpiness, and he's probably still the best and most productive engineer in the team, but to me this behavior is kind of immature and a bit unprofessional. On the other hand, the salary cap is not something I set so there is not much I can do there.

How should I handle this situation?