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As I often say, anyone can sue for anything. The real question is can one win?
It sounds to me that this customer had a problem probably for long before he ever encountered you. After all, his first two checks bounced. I bet he's bounced lots of checks. More than likely, his business was on the downhill path long before you ever posted the question, and will continue to be.
So, sure, he can sue you - but it's really, really unlikely he'll find an attorney to represent him for free. And it doesn't sound like he'll have the money for a retainer to pay someone an hourly wage. Perhaps he can try to take you to Small Claims Court. I doubt he'll win.
My best advice is "don't worry, be happy" - don't worry about getting sued until if and when it happens. Meanwhile, put everything regarding this incident into a file folder and keep it 7 years.