Chaim,
I would select yourTable into a cursor with an extra field nResult N(10,2) set the cursor to the datetime field order or field id order - your option , create 2 variables lnIncome N(10,2) and lnExpense, scan /endscan the cursor updating the nResult field.
A test with a table with 400 records on my pc 0.031 sec
Please report back if this is what you are after.
Please note there is also a SQL statement possible to achieve this runningbalance but I am not sure this works on VFP8.

Shalom

Koen