Edit the SR invoice and instead of crediting sales, credit the liability account (you can override the GL account on the invoice details tab)

Hi Jessica. So he wants the amount of the items he purchased to be added as an additional amount to the loan he made to the company?

He would like it to reduce the amount that the company would have to be paying him back essentially. 

Maybe I'm not understanding the situation, but is he wanting the payment for that service invoice to be applied against the loan?

He created the invoice, and wants the balance of the invoice applied to the loan to reduce the amount of what he loaned to the company, if that makes sense. I've been trying to wrap my head around it as well. 

Is it possible that I would just need to void the invoice and create an entry to reduce the loan amount? 

If there are any payments already applied you won't be able to void the invoice.  But you can issue a credit invoice and code that against the loan account (override the GL account on the invoice details).  Then apply the credit to the SR invoice to get it off your aging

There isn't any payments applied to it yet. It is still open, so that's why I was trying to figure out a way to mark it paid but post it to the loan account. 

Issue the credit memo like I listed above