You can take advantage of the Quoted Price Pricing Type and its flexible Final Price to apply a discount to a job. Here's how you can do it.
Step by Step:
Go to Projects and select the job you want to apply a discount to.
Select a workarea to apply the discount to. Make sure the Pricing Type is set to Quoted Price. Edit the Final Price to be the post-discount amount.
If you created this job earlier as a Time & Materials or Flat Rate job, you will need to edit individual item prices instead of the Final Price in order to achieve the amount that you want to charge.
To make the discount obvious to the customer you can edit the name of the workarea to include this information. For example, "($1,828.778 discounted to $1,600)". Or you could put it in the Customer Proposal and Invoice Description Notes field.
If you have any questions, send them over in the blue chatbox!