I don't work with billable expenses, and I'm not quite sure it would be appropriate in this situation either as I am not entering an expense through the bills/expense feature which would be billed.  I'll use the following terms to try to simplify the scenario (Trucker would be my client who is using QBO and Broker would be the "customer" that is being invoiced).

What happens is that Trucker hauls products for Broker, and Broker supplies a fuel pump that Trucker uses.  Trucker gets paid by Broker twice a month.  Broker determines the amounts that Trucker earned based on their scaling equipment (Trucker doesn't submit an invoice to Broker) and the amount that Trucker has used in fuel is summarized.  Broker submits a summary of both the fuel expense and the hauling income to Trucker and pays Trucker the NET amount of both transactions. 

Ideally, I need to be able to track that information in a single invoice entry in QBO, where I will track all of the following information
a) the income (no problem there)

b) the hst on sales (no problem there)

c) the fuel expense (no problem there)

d) the hst on purchases from the fuel summary (this is what I can't get to work)

I currently have created a temporary GL account for the transit of the expense.

The first entry would be the customer invoice that tracks the net amount receivable.  

a) Sales $1,000 + HST
b) Temporary Clearing Account  -$500

The second entry is a general journal entry that redistributes the temporary account.

Fuel Expense DT 500  (Inclusive of HST against HST on Purchases)

Temporary Account  CT 500

I dislike this workaround as it creates the need for 2 entries, increasing the risk of error and workload.