Hi Louis,
We appreciate your interest in Syncfusion products.
If you want to apply format to the Summary cell, you can specify the formatting in GridSummaryColumnDescriptor as the following code snippet.
GridSummaryColumnDescriptor summaryColumn2 = new GridSummaryColumnDescriptor("FreightTotal", SummaryType.DoubleAggregate, "Freight", "{Sum:#,###.00}");
Please refer the sample in the below link that illustrates the above.
http://websamples.syncfusion.com/samples/Grid.Windows/GGCSummaryFormatting/main.htmRegards,
Jisha