My employer offered an employee purchase plan (toward a laptop) - I paid for the computer monthly, they reimbursed me the same amount. I am paid by automatic deposit and never look at my pay stub. Now it turns out that they continued to make the payment for about three years past when they should have. The amounts were small, but added up and I now owe about $6900 (!). Any advice on how to deal with this... clearly I can't cough up that much money all at once.

See if they will merely take it out a small bit at a time, or ask if they would settle for perhaps 1/2 and let you settle for keeping the other half as their error. Since it was their mistake they should be more willing to work with you