I have just been testing the Multi-Buy Discount functionality and have noticed a bug.
Version 8.4 2018 - 07 - 26
Create a new Multi-Buy Discount record for an item
set Buy any Qty to 1
Fixed discount: 15%
Disc Type: Cheapest
Type: Buy any Quantity from Item Set
Add an item to the row matrix and in the buy qty column put 1
Now go add this item to a sales invoice. You will notice that if you add more than 1 of that item, the discount gets calculated incorrectly.
Instead of just the Multi-Buy Discount qty being changed to -2, the unit price of the discount item is also changed thereby effectively doubling the discount given. The unit price of the discount should not be changed when you add more than 1 of an item, only the Multi-Buy Discount qty should change according to the qty purchased.