Hi,
I need to modify the Amount Field of the sales invoice item.
Currently Amount = Quantity*Rate but the customer request for Amount is as follows 
Amount = Quantity*Rate(*Slaughter fees)
Can you please guide me how to do this change and where ?
-Kartive
I will recommend to dont do that!
You, have two ways to do that one without hardcore modification, and another with a minimal of modification.
Invoices have (Taxes and Charges) if it’s a Global Charge, include one line there!
If it’s per Item basis
Add one custom field for Slaughter fees in Sales Invoice Item.
When that field is modified, you can set again the rate using price_list_rate * slaughter_fees it’s the most organic way to go, and dont will make you stuck in migrations.