When one says Salam without Alaykum, does the receiving person of the greeting have to respond with Alaykum Salam or is just saying Salam sufficient?
It is obligatory to respond to the Salam as it was given, and it is recommended to add on it to make it a better response. If you were been greeted by (Salam), you must respond with (Salam) as minimum response, but adding Alaykum on Salam is better.
Wassalam.