We would like to create a separate product that represents a "discount" for the invoice. This would allow us to create a new product line item on an invoice which shows "-€25 Summer Discount". An example is shown in the invoice Sage itself sends for the Accounting Start package due to the signup promotion.
It is currently not possible to store a negative price for a product, as the UI restricts prices to be above zero. However, when adding a product to an invoice, you can then change the price from positive to negative anyway.
Being able to store this discount in the product instead of typing a minus symbol to each invoice would speed up our daily process.