customer wrote "paid in full" on check

What is the name of your state? Iowa

I have a customer, that disputed his balance, only wanting to pay for the locks, but not the labor hours it took for my company to install them.
he wrote a letter stating that, and wrote paid in full on the check-
I did write him back, and tell him I would accept his partial payment and that the balance would need to be paid.
he now writes me, that in accordance with the Iowa code, that i accepted a payment with "paid in full" written on it, therefore i agreed to settle for the less amount- and that the issue was now closed.
I didnt agree to any such thing, as i wrote him back thanking him for his partial payment.
and so far i cant find an actual Iowa code that supports this.
Is there any truth to his claim, and if so, is there any way around it?

Did you cash the check as it was issued to you with just your signature ?

If so, you technically agreed to that amount.

Kelly, what the person did is called a 'restrictive endorsement' (or 'accord and satisfaction').

In some states, the "Paid in Full" endorsement can release the debtor of any further obligation. Some states require that the debtor send you a separate letter declaring his intentions first, and finally, some states don't allow the "Paid in Full" to apply at all.

Luckily, your state (Iowa) is in the latter group. The endoresement isn't valid. This is found in the Iowa Code (§554.3206) at:
http://www2.legis.state.ia.us/IACODE/1999/554/3206.html

For more on this, go to:
http://www.carreonandassociates.com/restrictiveoffer.htm