Negative price item not working

Hi, wehn we add a negative price item (to give a fast discount via barcode), the discount is only the amount of VAT and not the full discount.
Example :
we make a hidden product with -2.5 € price, and we alwwasu use 21% VAT in belgium
when we scan the code in oour WePos, wehave a 0.43 € discount on the WePos, while in the WooCommerce order list, the discount was correct But the maount seen on the Wepos is not correct en thus, we cannot use this to do fast selling