It could be that he knew how much she means for Geralt and because he was the one who captured her.

But I rather think he never actually did save her. It's more likely that after all the beans were spilled Nilfgaardians no longer needed her and simply allowed Letho to release her or rather released both of them. More over they couldn't reveal that Triss was in their custody so they probably would have done it regardless.

It’s an interesting idea, but as Triss says to Geralt “he saved me”, it would imply that Letho really did save her.

HOWEVER, she could’ve been referring to the massacre that had just erupted, which is plausible given that she’s safe and with Letho. That’s the other thing, though: after being arrested and brought before those at the summit, how and why did Letho find and protect Triss? It does feel as though he has a genuine desire to keep her safe, but his motive is hard to determine.

I agree it's more of saving her during the massacre because she was close to Geralt. Also Letho wants to give her as a souvenir to Geralt when he finally meets Geralt for the things Geralt had done for him in the past( if that explains).