SA may not be the only login that can perform a backup.  Backup permissions default to members of the sysadmin server role and the db_owner & db_backupoperator database roles.  If there's a login in one of those roles that you know, use it to do the backup.

If you do a search on this site for "SA password", you'll find several threads describing how to gain administrative access to SQL Server if the SA password is unknown.

Greg