Updating TextBox

procedure Form1_OnShow (Sender: string; Action: string);
  begin
    Form1.edSaleTabSaleInvTotals.Value := Form1.tgSearchSaleInv.Columns[9].Footer.FormulaValue;
    Form1.edSaleTabPurchInvTotals.Value := Form1.tgSearchPurchInv.Columns[6].Footer.FormulaValue;
  end;


procedure CalculateTotal;

begin
    Form1.edSaleTabDifference.Value := Form1.edSaleTabSaleInvTotals.value - Form1.edSaleTabPurchInvTotals.Value;
end;

procedure Form1_edSaleTabSaleInvTotals_OnChange (Sender: string);
 begin
    CalculateTotal;
 end;

 procedure Form1_edSaleTabPurchInvTotals_OnChange (Sender: string);
 begin
    CalculateTotal;
 end;