Negative unit amount per item & Credit notes

[blanc6]

Describe the bug
The function for creating credit notes doesn't exist, so I'm trying to create invoices with negative unit amounts (which amounts to the same thing). However, there is a bug. A negative unit amount of -100 gives me a total on the line of -1...

Expected behavior
Negative unit amount should be equal to the negative total of the line.

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Please complete the following information:

• Crater version: 6.0.6

[lowbits]

Its a duplicate of issue-597