It is Dosvidanya, that's the actual way it is spelled in Russian, with an "O". At the same time you will most likely hear it as Dasvidanya, because it's just easier to pronounce, so russians say it as an "A".

I mean, it's the same as the dilemma about the two ways of saying, potato and tomato :)

By the way, Dosvidanya means, literally: Do = until and svidanya = meeting/date, basically until the next time we meet :)

As for the farewell part, I would say the closest in Russian is the parting sayings: Vsevo horoshevo = All Well (Wish you all the best) or Dobrova puti = Good/Kind Way (have a safe trip)

Hope it helps :)

By the way it's written separately, it isn't a one word - Do svidaniya - До свидания. It's used when you believe that you'll meet someone again, when you're not parting forever.