You *can* be sued in the sense that, unfortunately, it is very easy for someone to file a lawsuit, even a bad or baseless one, since the courts do not "prescreen" suits before they are filed. However, based on what you write, they do not appear to have a good lawsuit: without a written contract, there was an oral contract or agreement as to what you'd do and what they'd pay. To recover money, they (as the party suing) would have to prove in court (e.g. by credible testimony) by a "preponderance of the evidence" (that it is more likely than not) that the agreement was that you would bill them less than they paid. That may be difficult for them to do, since they *did* pay the amount you asked for--they gave you a check. If they truly thought the amount was wrong, they should have negotiated on the spot, only written a check for what they believed was the correct amount, refused to pay, etc.; by paying, they showed agreement with the amount, and it appears that this is a case of them after the fact wishing they paid less. However, the law does not care about after the fact "buyer's remorse"--if they agreed to pay what they did pay, which on the facts they appear to have, they can't later rethink that amount. Futhermore, if you did cater for them, there is no legal basis for getting all the money back: clearly, you are entitled, both contractually (per the oral agreement) and by the theory of avoiding "unjust enrichment" to be paid for the work you did; the most they can reasonably try to do (though as stated, the facts you describe appear to be against them) is to recover the difference between what they allegedly "should" have paid and what they did.