This gift is too valuable, I can't accept it - I'm returning it as-is.
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奉还 is a humble/honorific word - 奉 means 'to offer with both hands', so the speaker is symbolically offering something back with deference. Use it in formal letters, polite speech, and historical drama. The plain modern verb is 还 or 归还.
usage
The set phrase 双倍奉还 ('pay back double') was popularized by the 2013 Japanese drama 半泽直树 (Hanzawa Naoki) - its original Japanese line 倍返しだ! became a catchphrase across East Asia.
