Thanks for your interest in Syncfusion products.
We have analyzed your sample and we are little bit unclear with your requirement. Here we have modified the sample based on what we had understood from your update (i.e the negative values in decimal will be exported as zero). So in order to resolve this scenario, we suggest you to use the GridGroupingExcelConverterControl instead of GroupingGridExcelConverterControl in your sample. Could you please provide us with the below details,
· Provide the simple screenshots of your grid and exported file which shows the reported issue.
· Confirm us with whether reported issue is resolved by using GridGroupingExcelConverterControl.
These would be more useful for us to provide the exact solution at the earliest.
//To set the currency format.
this.gridGroupingControl1.TableDescriptor.Columns["ValueDouble"].Appearance.AnyRecordFieldCell.Format = "C";
GridGroupingExcelConverterControl converter = new GridGroupingExcelConverterControl();
converter.ExportToExcel(this.gridGroupingControl1, "Sample1.xlsx", new ExcelExportingOptions());
Please refer to the below UG for further references,