I assume, since aviad688 and wmarovitz both say שלום can also be used a goodbye, that 'Hello later' is just a poor translation into English, and should not be the provided answer. As a native English speaker, I agree that the phrase 'Hello later' does not make sense/mean anything.
The report flag for this sentence doesn't allow reporting on the thing that needs it most. I came to note that the other correct solution, "Hello, goodbye!" is also not good English and not said by native English speakers. Even busy people passing each other on the street don't do that. "Bye, see you later" or "Goodbye, see you later" is more like it. Other people have said one or the other, above. 25 November 2018