Paperwork on the sale of paintings

[ConSferArt]

Hello!
I have a very important question for the knowledgeable people.

There Artist, it is seller, which also serves to a third party ( the commission ) K omitentom.

commission is selling a picture of artist, receives a percentage of it , and passes the rest of the cost of painting to the principal . That , in turn, - artist .

Attention . question . What documents are made with the Principal and to the commission for the artist ? Is the sales contract ? Is there something else?

I beg for advice from knowledgeable people . Very grateful in advance to any prompts.

If you put the question in the wrong topic , please specify . to what is correct.