Ethical Dilemma - Seeking Advice

One of the downsides of being a person who works many part-time jobs is that sometimes I neglect to pay regular attention to my employers and their abilities to pay me on time. As a result several times last year my wife reconciled our accounts and pointed out that one institutional employer neglected to pay me as scheduled.

This was especially annoying because of the various excuses provided to me by the people who addressed my queries: "This is a special account that must be hand-posted" and "We just started a new software system" and "The recent changes caused our systems to get backed up" and "They pulled our accounts payable personnel and put them on a special project."

At one point last year they were three payments behind, and they finished the fiscal year by screwing me out of at least one - and possibly two - checks. When I complained, they put the burden of proof on me, insisting that if I could not produce all the pay stubs, they would not be able to tell if I was paid or not, since "the system has a lot of problems and we have no way to check on our end."

Of course, since I did not actually receive the checks, I could not produce the requisite pay stubs, right? I ended up just writing off the amount and vowing to watch this particular employer very closely.

Thus I was quite surprised when said institution (which shall remain nameless) accidentally cut me a check this month that I technically did not earn. So I am now faced with a dilemma: should I keep and cash this check, an amount in the hundreds of dollars exactly equal to the amount I was shorted last year (assuming they only screwed me out of one check), or should I take the high moral ground and return the check despite the earlier failure to pay all monies to which I was entitled?

My inclination is toward the latter, and to send it to the highest-ranking officer as proof of the general failures of the accounting systems. Yet I am still irritated at the poor service and treatment I experienced last year in dealing with the less-than-helpful office staff, and I could always use the extra cash to buy a stone sink or something.

Thanks in advance for your comments, and I look forward to reading your responses.