books/schemas/app/SalesInvoiceItem.json

{
  "name": "SalesInvoiceItem",
  "label": "Sales Invoice Item",
  "extends": "InvoiceItem",
  "fields": [
    {
      "fieldname": "isFreeItem",
      "fieldtype": "Check",
      "default": false,
      "hidden": true
    }
  ]
}