You can apply credit from one invoice to another upcoming invoice. So, if a user has a negative invoice, you are able to apply their credit to one or more invoices by:
Navigate to the negative invoice in the Invoices page
Click the three dot menu in the top right hand corner of the invoice
Select Apply Credit
Indicate the amount that you would like to credit to one or more invoices
Click Apply Credit
Alternatively, you will be able to navigate to a positive invoice of a user and click Apply Credit in the top right hand corner of the invoice.
Need more info on Invoices?