An employee I hadn't talked to emailed me saying I needed to describe the purpose of the payment, provide proof I own the money
It's normal question from bank (KYC, AML). If you and your partner have company then what's problem to provide them proof that you are the owner of that money?
If your partner is private person - then you should sign legal document. If he can to send 100K without any legal document to you, maybe he can send all that to us? ;-)