[perlgeek]

I guess you could have a stored procedure that checks for balanced transactions before inserting (or before committing, at least), with SECURITY DEFINER, and not give anybody else permissions to insert or modify the transaction lines table.

But yes, that is a downside, and if I were to write such a thing I'd make sure to have at least two mechanisms to avoid / detect errors (like, one validation in business logic and/or stored procedure, plus regular monitoring for transactions that don't add up to zero).