This is a slightly different question from How to differentiate Sie (they) or Sie (you)?.
The major question here is when I am directly speaking with someone whom I address as Sie, how do I go refer to a different they, with whom I would normally use “sie”.
For instance, how do I make it clear that:
Sie haben mir gesagt, dass Sie einen neuen Beleg unterschreiben müssen.
that the second Sie is actually Sie, and not sie? It would be a major annoyance to have to make the references explicitly every time:
Die Kassierer haben mir gesagt, dass Sie einen neuen Beleg unterschreiben müssen.
Is there any more convenient way to make this distinction clear in conversational German?