I have to keep a track of the sales and see if i get the payment. Payment is
received according to the invoice numbers. Eg invoice no 1 dated 1/1/2005.
payment will be expected after 45 days, say 16/2/2005. so now when i feed in
the invoice number i want it to find in the whole list of various invoice
numbers the particular invoice no, tally the value, if not true then mention
the +/- value.. so how do i do that

Take a look at the SUMIF() function. For the plus / minus value, you
could return (in a different cell) the amount expected, and subtract
the expected amount from the amount received.