I thanked him for the offer but turned it down.

我感谢他的好意，但拒绝了他。

《柯林斯英汉双解大词典》

I thanked him for his help.

对他的帮助，我向他致谢。

《新英汉大辞典》

I thanked him, but what I got from him was much more than just an ice cream.

我感谢了他，但我从他那里得到的比仅仅一个冰激凌多很多。

youdao

When I finally netted my partner, a 20-pound rainbow trout, I thanked him for the fun, unhooked him and tossed him back.

WSJ: Learning to Fly Fish��And Fend Off the Grizzlies | Traveler's Tale

So, I thanked him for his interest and apologized for my oversight.

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Following our work together, at the G20, the Prime Minister and I discussed our close cooperation in APEC and I thanked him for the hard work that Japan has done in preparation for our meetings here.

WHITEHOUSE: InterContinental Yokohama Grand Hotel, Yokohama, Japan