Why does Roy Mustang lose his eyes when he goes through the GATE of Truth in Fullmetal Alchemist?

In the manga and anime series The Fullmetal Alchemist, Roy Mustang lost his sense of sight when he was forcefully made to open and enter the portal by Pride and Wrath. When he entered the portal, he saw the Truth and became the sacrifice to swallow God and absorb all the Amestris’ souls. Because of that, he went blind.

But in the end of the series, he was offered for his sight to be restored with the help of the philosopher’s stone. It was a success unlike the other characters because Mustang was not punished by the Truth rather he was forced to be the sacrifice, technically he was free from sin.

Whoever opens the Gate will be revealed the truth and have something precious or essentially ironic taken from them. In Mustangs case, he wanted to be the leader and fullfill his vision for Amestris to be a just nation , military nonetheless. Which is what Truth took from him, how can a leader lead when he has no vision to see the future of his country come to fruition.

As Mustang had already seen the truth, he had to pay the price for it. That is the law of equivalent exchange. As for Truth's intentions, I would say that truth does not have any intentions. Truth is the unbiased reflection into oneself.