would like the option of locking the transaction like sales orders, invoices in such a way that a user wont be able to change things like quantity, add items, etc in the Item table even if he/she is given the power of editing Invoice.

maybe create a new permission field under edit role (User & Roles) such that we can restrict which user can edit the contents of Item Table so that he/she wont be able to change the amount of the Invoice etc but can enter the data in other fields of the invoice