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.