Hi cfox,

You can unlink the billable expense from the voided invoice by clicking the Remove link. Let me show you how. 

1. Click Sales on the left menu. 
2. Select Customers.
3. Type the name of the customer on the search bar.
4. Select the customer.
5. Click the voided invoice. 
6. Click Remove to unlink the billable expense.

Once done, the billable expense status will now show as open. You can now create a new invoice and link the expense transaction again. 

I have this link in case you have other questions about QuickBooks Online (QBO): Money In, Money Out.

Fill me in if you need anything else. I'd more than happy to help.