Customer Outstanding Balance in Sales Invoice

frappe.ui.form.on('Sales Invoice', {
	refresh(frm) {
	cur_frm.cscript.customer = function(doc) {
	return frappe.call({
  method: "erpnext.accounts.utils.get_balance_on",
  args: {date: frappe.datetime.nowdate(), party_type: 'Customer', party: doc.customer},
  callback: function(r) {
  	doc.cust_os = format_currency(r.message, erpnext.get_currency(doc.company));
  	refresh_field('cust_os', 'accounts');  //cust_os is a custom field
  }
	});
}}})