Is there a federal rule that generally requires pay when a worker is sent home early?

The U.S. Department of Labor material reviewed does not state a general Fair Labor Standards Act (FLSA) requirement to pay a worker merely because the worker reported for a scheduled shift and was sent home early. It explains that if an employer makes “show-up” or “reporting” pay in that situation, the payment may be excluded from the federal overtime “regular rate” when it is infrequent and sporadic. That is an overtime-calculation rule; it does not itself say when a payment is owed. [1]

A state or local rule may use different terms and may matter to whether payment is required. No jurisdiction was supplied here, so this review cannot explain the policy behind a particular reporting-time or scheduling-pay rule or say whether one applies.

To check a particular rule, identify the state and any relevant locality, then consult the current official labor-agency page, statute, regulation, or wage order for that jurisdiction.

This is general public information for review, not advice about a particular job or situation.