So on the Hashashin level up event page it says for hitting level 60 you get a 1KG gold bar, which is 1 billion silver. In game however it says you only get a 1,000G gold bar which is 100 million. So, which is correct?

That is most likely a typo as in game at lvl 60 it states a 1,000G bar. But it could be considered a bug in the ingame rewards so someone could get confused. They should fix that typo.

The level 60 reward does say a 1kg gold bar, however 1kg=1,000g. A 1g gold bar is worth 100,000, and 100,000 x 1,000 = 100,000,000. So there is no typo, a 1kg gold bar is worth 100,000,000 silver