It really does not matter what business records he retains internally since these are the records upon which they must rely to muster a defense in a collection action. If they do not have a cancelled check to prove they have made payment, it is a simple matter of their requesting a copy of the check from their bank. Most banks keep these copies available for 7 years.

File the civil action against the delinquent client. The only reason I can think of for their not being concerned might be that they have no assets, and may be judgment proof. However, in these scenarios I have an awful lot of fun forcing a delinquent client to produce documentation in both pretrial and post trial discovery. So far the only time I have ever not been paid was when the delinquent client filed for bankruptcy. Otherwise....let the games begin.

I find that unconcerned delinquent clients become very concerned when you garnish their accounts receivable from their clients.