We always had one "Donor" in the system called "Anonymous" for situations like this. When the donor is unknown, we used this donor. When the donation category is unknown (like, if there hadn't been a donation slip in the envelope but only cash) then the whole amount is put into tithing. As far as I know, the policy in the past said to handle it this way (someone can correct me if I'm wrong and/or not current). If you can find out who the real donor is then you can correct it but, otherwise, the money needs to go into the system rather than sit in a drawer waiting for clarification. Maybe you'll find out during tithing settlement when a donor thinks there's an error on his paperwork. Maybe you simply won't find out who really made the donation. (For a while, in our ward, we had a donor who wanted to remain anonymous. Although I knew who he really was, he still wanted to be anonymous on the records, so I let it be that way.)
My real name is Greg Anderson.