Thanks! yes it was a running total. If anyone is interested here is
the resolution:
Create a variable
In product header add formula (and suppress so not visible)
whileprintingrecords;
global datevar NegDate:=date(1900,01,01);
// This just rests dates date for each product
In detail add a formula
whileprintingrecords;
global datevar NegDate;
If Negdate= date(1900,01,01) and #RT Balance < 0 then
Negdate:= datefield;
// By testing for reset stops date being over written by subsequent dates when balance is still negative. YOu will need to creat a running total for balance which is reset on each change of Product group.
In group footer add formula
whileprintingrecords;
global datevar NegDate;
And that worked perfectly.