Electromagnetic waves carry linear momentum. So when they strike a solar sail (or any reflective surface), the object experiences radiation pressure and net energy gain.
The reflected EM waves must then have less energy. Does this result in a redshifted wavelength or a decrease in expected photon number? I'm inclined to say it lowers photon number (or in classical E&M, a decrease in field amplitude). Curious if there is an authoritative statement on the matter.