How can we write a budget approval rule that triggers for task to task budget changes?

Summary:

We have a need to trigger a budget approval when >500K has been moved from Task 1 to Task 2 in the same project. However, we don't see task as a source in BPM. How have others accomplished a budget approval when dollars are moved from one task to another in the same project.

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Version (include the version you are using, if applicable): 26D

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