We don't know who donated this, but thank you very much for your kindness.
We don't know who the donor was, but thank you to him or her.
この場合、次のような言い方ができますよ。
ーWe don't know who donated this, but thank you very much for your kindness.
「どなたが寄付してくださったかわかりませんが、ご親切にありがとうございます」
ーWe don't know who the donor was, but thank you to him or her.
「寄贈者がどなたかわかりませんが、ありがとうございます」
ご参考まで！
カナダ 
