PostgreSQL: restore a record from a backup
On way would be to restore only the relevant table to a new empty dummy database. Then find rows with standard SQL tools. Not sure if you are restricted by Heroku in any way. On the shell, as postgres user (or any user with sufficient privileges) first create an empty staging DB:

createdb -T template0 dummy


If the DB isn't too big and you have plenty of disk space, just restore the whole DB for simplicity. Else, selectively restore tables:

pg_restore --dbname=dummy --schema=public --table=foo my_backup


Details in the manual.

Or if it's just 1 or 3 records I would open the (default plain-text) backup with vim or your editor of choice, search manually, copy selected rows (lines in the backup) to a new file my_copy_file.sql. If you are sure about the data you can COPY into existing table directly, rows will be added.
If you need to manipulate data first COPY to a new, empty table. Create it first if it's not there, yet:

CREATE TABLE staging (Like foo);
COPY staging FROM 'path/to/my_productive_db`


Then proceed from there.