question about query

Hi,
    i am trying to obtain from the database what we have been doing in a excel
sheet from some time.

I have some tables where i store the documents associated with our customers,
invoices, payments and others, what i need to get from this tables it is a
movement log where i can get what payment it is pending from this customer after
the change in this row, for example:

movement              |   qty      |   pending
invoice N-111         | 1000.0 |   1000.0
payment 1 N-111   |   200.0 |     800.0
payment 1 N-111   |   150.0 |     550.0
invoice N-112         |   350.0 |     900.0

how could i calculate this pending column that does not exists in the database
and i need to be based on last result row? I would like to get with raw SQL but
the need to use any procedural language would not be a problem.

Thanks!

Regards,
Miguel Angel.