If you're looking for a pre-built double entry accounting Postgresql schema that provides database-level integrity checks:

https://github.com/adamcharnock/django-hordak

I created and maintain this, along with a couple of others. It is built for Django (so great if you're using Django), but extracting the schema wouldn't be too hard. It also has MySQL support, but the integrity checks are more limited.

(Side note: I'm a freelancer and available!)