Hi, I'm Robert and I'll be helping you with your financial software question.
The better way to do that is to set up the owner as a vender. You purchase the rent from him for 800 and sell it to the home owner for 1000.
you might want to look over this page on how to set up a consignment which works the same way where you don't actually own and sell the item but hold it for the seller and then take the entire price from the buyer and give the part minus your commission to the seller.
you can add an item to the invoice for 800 for homeowner
or create another invoice for the same rent for 800 with item name - owners share or something descriptive like that.
apply the credit from the extra 800 to this other invoice or new item on the old invoice