How to use negative price and positive quantity in sales order

Hi, all!

I have an application that exports sales orders to SAP b1 database - I use the DI API pl36 for that purpose. What I want to do is to export some of the sales order lines with negative totals. I found out that if I set the quantity field of an order line to a negative number and then set the "single item price" of that line to a positive value then I get in a negative line total. Then I tried to set line quantities to be positive and single prices to be negative because negative quantities in a sales order sound strange. It did not work - to my amazement the order lines once more had negative quantities and positive single prices (SAP b1 somehow switched the negative sign).

How can I achieve that quantities stay positive and prices are negative?

Thanks in advance

PS: The SAP B1 client application itself does not allow to set a negative single price for an order line - tried it and got an error message saying "Invalid total". Are there any settings that block this behavior?