Problem
Sometimes, a bill is created outside of a project but needs to be attached to it. If the bill has been reconciled and exported, attaching it to the project becomes a challenge.
Solution
Attach a bill to a project after it has been reconciled and exported
Bills need to be created from the project page to attach automatically. If the bill was created elsewhere, like from the Accounting tab's bills section, it won't show under the project. Unfortunately, once a bill is reconciled and exported, it cannot be attached to the project.
The workaround is to create a return against the incorrect bill and then create a new bill from the project page. Best practice for the future is to only create bills from the project page.