You can handle the CurrentCellInitializeControlText event and do it there.
Private Sub gridControl1_CurrentCellInitializeControlText(ByVal sender As Object, ByVal e As GridCurrentCellInitializeControlTextEventArgs)
If e.RowIndex = 1 And e.ColIndex = 1 Then
Dim d As Double = e.CellValue
If d < 0 Then
e.ControlText = d.ToString()
End If
End If
End Sub