Does installing Yosemite on a macbook shipped with El Capitan void the warranty?

The IT department at my work have an image they put onto work computers. We are hiring a new designer and they are refusing to purchase a new computer as they state that their image can't be put onto a new mac, shipped with El Capitan.

When asking if they can simply roll back the OS X to Yosemite, they have stated that this would void the warranty on the machine. Does anyone know if this is fact?