Custom value in matrix

I want to set the total in a delivery document, so I have this code:

if (EventEnum == SAPbouiCOM.BoEventTypes.et_LOST_FOCUS && pVal.ItemUID.Equals("Rental") && pVal.FormTypeEx.Equals("140"))
                {
                    form = SBO_Application.Forms.Item(pVal.FormUID);
                    form.DataSources.DBDataSources.Add("OITM");
                    SAPbouiCOM.Matrix matrix = (SAPbouiCOM.Matrix)form.Items.Item("38").Specific;

                    SAPbobsCOM.Recordset rs = (SAPbobsCOM.Recordset)SBO_Company.GetBusinessObject(SAPbobsCOM.BoObjectTypes.BoRecordset);

                    int daysQtt = int.Parse(((SAPbouiCOM.EditText)form.Items.Item("Rental").Specific).String);

                    for (int i = 1; i < matrix.RowCount; i++)
                    {
                        String s1 = ((SAPbouiCOM.EditText)matrix.Columns.Item("1").Cells.Item(i).Specific).Value;
                        String unitPrice = ((SAPbouiCOM.EditText)matrix.Columns.Item("14").Cells.Item(i).Specific).Value;
                        float preco = parsePrice(unitPrice);
                        string sSQL = "SELECT U_rentable FROM OITM WHERE ItemCode = '" + s1 + "'";
                        rs.DoQuery(sSQL);
                        rs.MoveFirst();
                        SAPbobsCOM.Field f = rs.Fields.Item(0);
                        double finalPrice = 0;
                        if (f.Value.Equals("2") || f.Value.Equals("Y"))
                        {
                            finalPrice = daysQtt * preco;
                            //Fails in the next line
                            ((SAPbouiCOM.EditText)matrix.Columns.Item("21").Cells.Item(i).Specific).String = daysQtt.ToString();
                        }
                    }                    
                }

I don't understand why I can't change the value because in the sales order I'm able to...

Is there any kind of restriction or something?

Thanks in advance