As far as I can tell, Numbers uses the standard hyphen/negative symbol for the minus sign (ASCII code 45) when it exports negative numbers to CSV. But we have noticed in the past that there are regional differences in how Numbers works. Something that works perfectly fine in the US region might not work the same in another region.
As a test, do you have another spreadsheet app (Excel, Google Sheets, etc.) you can use to import the CSV file to see if negative numbers get imported as numbers, not text? The test is to be sure they import as numbers, not text. That will rule out it being a problem with negative symbols in the CSV. If it passes that test, the problem is something about Fortnox.
Does Fortnox require negative numbers to be formatted with parentheses as (519,00) vs -519,00 ? That would be strange but it is a possibility. You could try formatting all numeric cells to use parentheses for negatives. The exported CSV will/should also use parentheses. As a test (before messing with your Numbers document), you can edit the CSV file you have now and change one or more of the negatives into that format and see if those numbers import correctly into Fortnox.