In an outsourced Scrum project how can we know the end-product is of the same value of what was originally promised by the contractor (noting the features might be different due to re-prioritisation of the Product Backlog) ?
The track record of outsourced software development is so bad that I would nearly always bet against getting the expected value and acceptable total cost of ownership (TCO) in the long run.
Are they really doing TDD?
Your vendors may cite their Scrum certifications as evidence they are Agile. If they are not doing Test Driven Development (TDD) (or something similar) the code will not be maintainable. How do you know the effectiveness of their technical practices if you’re not doing it with them? By now you know the tools that try to measure test coverage and code quality are easily gamed.
You lose the knowledge
Also, please understand that software development is knowledge-creation work. The value produced is the knowledge, not just the ones and zeros the vendor delivers. Outsourcing leaves valuable knowledge with the vendor instead of with your agency.
Expensive, late, and defective results
Over the years I’ve seen the outsourcing of software development attempted with various federal, state, and local government agencies. It nearly always made me cringe. It is not just that the long-term costs are so much higher than an agile approach would be. It isn’t just that it’s slow. Perhaps worst of all, the solutions have more defects than you’ll be able to find in time.
Try this instead
The only time I’ve seen this work reasonably well was when we mixed the vendor contractors in with the government employees on the same teams at the government location. This brought its own problems, but was still better than the typical project-based approach. See also FBI Sentinel, where the FBI finally got results by bringing a much smaller team right into the building, working with them directly each day. Small and skillful – connected directly to the customers and users – beats huge and clueless every time.
If you have no other choice but to use outside contractors, please at least read the Agile Contracts Primer to learn more sensible ways of doing it.