First you must set up the Service to allow negative entry.

       

Then, when you add the Service to the Job, select an Entry Mode of Negative Entry.

Enter a (positive) Quantity. When you tab off the Quantity field the value will turn negative and allow you to include a negative amount on the Invoice.

Note: It is not possible to have a negative Invoice or you would effectively be raising a credit and you would need to use a Credit Note to do that.