Here are a few
Lügen haben kurze Beine
Ehrlich währt am längsten
Wer einmal lügt, dem glaubt man nicht, selbst wenn er mal die Wahrheit spricht.
I'd be really careful, though, of directly accusing someone of lying. That could bring you even more trouble, I'd think. Something like "Folgende Äußerungen entsprechen nicht die Wahrheit" might be better.
Thanks, the lie was about my ex-employer's claiming that I was not at work on my last day. But in fact, I had 2 meetings with 2 different managers at my client's office on this last day, as well as been sat at my desk working on my handover document. That's why I accuse them as lying, because it simply is utter bullshit what they conveyed to their solicitor.