I am inheriting point_of_sale.OrderReceipt

When they are credit notes, the information on the ticket comes in a negative value. Due to country regulations, I cannot present a ticket with negative values. This is why I want to make the modification so that these values ​​are positive, but in the way that the lines are now presented. It doesn't let me achieve this in a simple way with previous versions

any idea how solve this ?

this is the code from original ticket