I have solved exactly the same problem with the following code: (except that I have the same field for debit and credit).

In the debit/credit cells LostFocus:
Thisform.txtSumma.Refresh()

In the txtSumma.Refresh: (tVerif2 is my grid table)
LOCAL lnRecno

if reccount('tVerif2') > 0
select tVerif2
lnRecno = recno()
sum belopp to this.value &&belopp is my debit/credit field
if lnRecno <= reccount('tVerif2')
go top
else
goto (lnRecno)
endif
endif

This works fine for me... I know that with this procedure I do a lot of code execution, but it dosen't matter becouse it is fast enough so the user never notice it and never get interrupted with his work.