Sales Invoice (Charge Item)

Deleting the line amount sets the line discount to 100%, any changes to the unit price or quantity are then irrelevant on this line from a total price perspective as you are offering 100% discount on the line.

Alter the unit price, not the line amount, this is a calculated field, and altering it changes how other fields are set.

With the item type it reads the possible prices and discounts and therefore if you change the Qty it will re-read the possible prices and repopulate, however if you just alter the unit price the line discount remains applicable and the line amount will still be 0.00. As the item charge does not have price breaks and prices set there is no initialisation of the pricing structure, so the 100% discount remains applicable whether the quantity or the price is altered. If you are telling me if you leave the quantity the same and alter the price and at this point the discount is resetting I would say this is a localisation or a modification you have here as it is not standard processing.

Indian Localization includes "Structure" which can be used to bill extra charges on the goods sold in the invoice.