If you need to return items purchased for a job or need to enter a credit from a vendor against a job, you can do this in your catalog by using the Allocate Materials function:
This will prompt for the job as well as which items you would like to allocate to the job. Putting in a negative number will represent an item returned to a vendor and remove the cost from the job:
This does not sync with QuickBooks so it will need to be entered into both situations separately.
If you'd like to enter a vendor credit that will sync to Quickbooks, you can log a negative bill as shown here.