What is the ruling on an employer stipulating to his workers that any manual labor they perform for him shall be free of charge?

Scholars stipulate, for the validity of ijarah (definition) (leasing/hiring), that the rent/wage (ujrah) and the work (amal) must be known. Consequently, the absence of a specified ujrah and the uncertainty of the amount of work they perform—whether in terms of a specific duration or a specific quantity—leads to the invalidity of the ijarah. Therefore, the transaction must be reviewed, and the amount of ujrah and work must be specified, in accordance with the hadith (definition): "Whoever hires a laborer, let him name his wage," and the Prophet's (peace be upon him) prohibition of hiring a laborer until his wage is made clear to him.