You made a deal with him to accept the $255 refund. He sent you an extra $105, for reasons we don't fully understand. Was it somehow part of covering up a scam? Was it an overzealous attempt to be fair to you? We don't know. All we know is, you paid the agreed price for the bike, and made a deal to accept $255 in compensation for damage.
Send back the $105 and be happy. The rest is between him and UPS.