If the error is in your favor (e.g. they forgot to take the downpayment from you), then while you would have to pay if they remember and seek to collect it--you have to pay the full amount if the ask for it, even if they forgot to collect it initially--you are under no obligation to voluntarily offer it to them; you could count yourself lucky until and unless they seek the money, at which time you'd have to pay it to them.