Hi,
That's not how you make a discount normally. The "retail price" field is for the MSRP:
en.wikipedia.org/wiki/List_price
So You should leave it empty.
In the price area, instead of entering 40, enter 55. Then, in the menu Orders>Discounts, create a new discount, assign it to your product, and enter a flat amount discount of 15.
That way, the final price of the product will be 40 and if you create a product listing menu item with the "only discounted products" setting turned on under the Products options tab of the menu item, this product will appear in the listing of that menu item.