The Messenger of Allah sallallahu ‘alayhi wasallam
said: “Oh people! Worship Allah, The Beneficent, feed His servants, and spread salam much, and you will reach Heaven in peace [and safety].” (al-Tirmidhi)
Two points are certain
1. Assalam o Alaikum will give Rewards ranging from 10-30 and it is Sunnat of Prophet.
2. Khuda Hafiz is not from Quran,Ahadith/Sunnah Rather it is custom/culture. So no reward is for Khuda Hafiz.
For a Muslim Prophets words and Sunnah should be more important than custom and culture.And we should follow it.Those who will say khuda Hafiz will miss the 10 to 30 Rewards.
Wheather it is permissible to say Khuda Hafiz either in place of Salam or addition to Salam ??????
Hakeemul Ummat Hadhrat Maulana Muhammad Ashraf Ali Thanvi (qudduhus sirruhu) mentions,
At the time of departure some people instead of “Asalam ‘Alaykum” say only ‘Khuda Hafiz’. Although this is changing of Shari’at, which is a big sin. Instead making the habit of saying ‘Khuda Hafiz’ after Salam is even forbidden. (Ahsaanul Fatawa, vol. 1 pg. 385)
[Taken from "Aghlatul-Awaam (Awaam ki Ghalat Masa'il)"]
(It is opinion of other scholars also that if you tell Khuda Hafiz compulsarily in addition to assalam o alaikum it is akin to addition to deen and if you say Instead of Assalam o Alaikum it is replacement or deletion in deen.So not permissible.)