Invoicing question

Hi.. I am trying to set up my first invoice for payment. I have the items set for what we 'sell' but I need to set up a item for what we 'buy' so that the invoice balances out with the total due being the outstanding amount after the credit. 

For example, if we sell $10,000 worth of T-shirts to our customer and they buy $4,000 worth of shorts from us in the same week, How can i set up the invoice so that the $4,000 is credit as a purchase from us and the total due will only show as $6,000 but it will link the $4,000 to our expenses.

Hopefully that made sense!