The NumberFormat field on the Excel Buffer table is used to define the formatting string that will used for any given cell in Excel. The field is currently 30 characters long. The longest "standard" custom format in excel is 49 characters long and is _-$* #,##0.00_-;-$* #,##0.00_-;_-$* "-"??_-;_-@_-

This format produces a leading minus sign, dollar sign, comma separated thousands, always two decimal places and dash for 0.

The format string also supports the ability for conditional colouring (such as making negative numbers red). Please increase the size of this field to allow more options. At the very least it should be increased to 50 characters, but ideally it will be increased to 250.