How to create a product with a NEGATIVE price

-- HikaShop version -- : 2.6.4 AND 3.1.1
-- Joomla version -- : 3.7.4
-- PHP version -- : 5.6.31

Reposting this as our comment in an old thread was not answered. ( www.hikashop.com/support/forum/product-c...ce-value/275022.html )

We need to know how to do this under two different versions of HikaShop. Our client's highly customized site is currently running 2.6.4 today, and we'll be updating it by end of Q3 to HikaShop v3 series. Therefore, we want to understand how to handle this in each, in order to code most effectively for the future upgrade.

Here is the end goal. We've built an admin checkout so an admin can dynamically create/assign orders to users so the admin can take phone orders as if they are the customer. We want to dynamically create an order discount PRODUCT on the fly with explanation that an admin can apply as they are on the phone. One solution we came up with would be to create a couple of order custom fields then dynamically add an admin only product to the cart during checkout with the explanation as one custom field on the item and using the custom price plugin to add a negative product price (or positive if they want to charge extra for something like extra fast processing, etc). They can't use coupons for this as they want to have this ability on top of coupons.

We need to know how this can be handled in both versions, please.