The data is stored in the database with different signs. Returns are stored as being positive, even though they should, strictly speaking reflect negatively on a company's balance sheet.

Taxes, on the other hand, are stored with the appropriate sign, because the table is shared. So taxes on invoices are positive, and taxes on returns are stored as negative.

We want to present to clients (enyo and otherwise) the "incorrect" version, where the numbers look positive. So the taxes need to be inverted. This is not documented on the client, but the client should not need to know how this sausage is made.