How best to document a photo license transfer to clients?

Hi Sheena, thanks for the response, but I don't see anything on that page that specifically deals with clients.  There's definitely nothing that addresses my question.

Maybe I can rephrase my question another way:

If someone from Adobe shows up at my client's office next week and asks them to prove that they have the legal rights to use that image, how do they prove it?

I understand I can buy a license on behalf of my clients.  There are many support pages, as well as forum posts, that explain I can purchase licenses for client projects, and that I have the right to transfer that purchased license to the client.  The licensing documents are also clear that I am allowed to do this.

My question specifically is how do I legally document this transfer. 

I thought I would include the Adobe invoices with the invoice to my client, but as far as I can tell, the Adobe invoices themselves do not include the image number.