Is just さようなら acceptable as a 'goodbye' in 敬語, or is there a more formal version?
さようなら is used mainly by school children, but adults use this less commonly in everyday conversations. Here's the list of possible expressions:
- お疲【つか】れ様【さま】です : Typically used after work. This is only polite enough to say goodbye to your colleagues. Don't say this to important external guests.
- (お先【さき】に)失礼【しつれい】します: Typically used when leaving (and entering) an office, conference room, etc. You can use 失礼します (without お先に) at the end of a phone conversation.
- (お先に)失礼いたします : Even more polite one with a humble form.
- ごきげんよう : Very polite greeting, which is described elsewhere. It's unlikely that you hear this in a business setting.
And there are shortened forms: 「お疲れ！」「お先に！」 These are no longer considered as "polite", but are very frequently used, instead of さようなら, among colleagues who know well each other.
You can always say
失礼します say goodbye or to excuse yourself (lit. "I'm being rude (by leaving))". This is probably the most common. There is also
失敬します, but I've heard this is rather old-man-ish and haven't heard anyone use in normal conversation.
If you're not in a work environment, maybe you could use それでは
If you want to go old school and talk like you're out a manga, you can also say さらばだ.