Yes, you can assign a job to an existing invoice. If your business creates the invoice first, waits for payment and then does the job, then you can assign the job to an invoice that’s already been generated.
This can be done when creating a job or when editing an existing job, use the drop down in the Financials > Invoice to select the invoice that you would like to be attached to the job;
To do this through the App, when creating a job or when editing an existing job > Invoice to select the invoice that you would like to be attached to the job;
An invoice can also be converted to a job.
In the App access the invoice > edit > convert to job (bottom row of the invoice), the new job page will appear to create the job with the invoice attached.
In the Web Portal access the invoice > edit > convert to job (bottom right of the invoice), the new job page will appear to create the job with the invoice attached.