Sayounara is not the everyday goodbye. It carries finality — closer to farewell than to see you later — which is why colleagues and friends almost never use it with each other.

What they use instead: otsukaresama desu leaving work, ja mata to a friend, shitsurei shimasu leaving an office or ending a call, gochisousama deshita leaving a restaurant. Each belongs to a situation, and using the right one is the clearest signal that you have paid attention to how people actually speak.