Investing in client's company by taking a partial payment

We have a client doing a podcast. We are helping the client expand the reach of the podcast via Social Media and other activities, including finding advertisers/sponsors. We have decided to invest in the client's company. We bill the client as if we are going to collect the entire amount (invoice for full amount). The client makes a partial payment on the invoice and the rest of the amount is intended as our investment in the client's company. How do I book the invoice amount left over after the client's partial payment?

I would like to track the investment in a chart of accounts entry but don't have a clue how this is done to show full credit for the invoice (as if we received full payment) and the amount left over after partial payment (expense? credit? other? special chart of account entries?)

Inquiring minds just gots to know! Thanks in advance for any advice on this issue.