Negative Discount Value Error

If you receive the following error:

"Negative Discount Value - Please remove negative discount value before syncing to Xero"

First enable the Discount (Disc %) column you via View Options to see which line item(s) have negative rates applied:

With the Discount view enabled, you can see for example below that -25% has been applied:

This is caused when the Quantity * Sell Price is less than the Sell Amount, for example:
Quantity: 1
Sell Price: $600
Sell Amount: $750

The difference being $150, so a negative discount of 25% has to be applied to get the Sell Amount to $750.

If the desired Sell Amount is $750, then the Sell Price needs to be set to $750 (from $600).