It is my take that a transaction is just an action. And we all know that every improv scene is not about the action; it's about something else. The problem comes when all we do is just play out the realism of a transaction; once the action is completed there's no compelling reason to stick around in the space. Remembering all that, once we focus on something else, anything else, during that transaction, we'll be fine. The second problem comes when we realize we're in the middle of a transaction and, having been told that they are an awful no-no, we panic and scramble to inject something into the scene. So the thought process is: always focus on something besides the transaction, and if you are acting one out, don't panic.
So what else can we focus on that can put us in a strong place? A few months ago I did a workshop with one of my groups during which we only played teaching or transaction scenes. Maybe it was a function of the way we like to play, but our scene naturally steered towards character. We rarely got around to completing a sale. Every scene had a very interesting character inhabiting the role of both the vendor and the customer. Challenge yourselves to play interesting characters; they can exist anywhere. Spending a day on that armed us with our weapon when confronted with transactions; you might find your own weapon.
It'll also be useful to brainstorm the other kinds of situations that might occur in a workplace. Breakrooms, lobbies, stockrooms, offices, salesfloors, those are just places. All of life can happen in a workplace. That couple could have just picked the tire shop floor room as the place for their break-up. That boss's son might have just picked this lunch hour to call and come out of the closet to him. Etc. etc.