No it is not open source.
Scripting is supported, via customisation
- Custom Fields: Use custom fields | Add an image to a custom field | Add custom fields for individual line items | Move a custom field on transaction forms
- Themes: Change the look of forms with themes | Customize themes
and Batch operations
- Batch Operations: Use Batch Create and Batch Update functions | Use the Batch Delete function
For more advanced customization search for localisation
An overview of the prior iteration is described here Localisation: GST/VAT worksheet programming guide
Manager has an API, search for API on the forum Search results for 'Api' - Manager Forum