Relevant UCC sections follow.
Disclaimer. While IAAL, I’m not your lawyer, and probably am not one in your state. This is general information and not legal advice. For that, see a lawyer licensed in your state.
Summary. Bank must recredit your account for the $800. The check was altered, and therefore not properly payable. Bank has a better argument with respect to the $160, because all necessary signatures were present. GIve formal notice in writing to the bank, though.
PART 4. RELATIONSHIP BETWEEN PAYOR BANK AND ITS CUSTOMER
(810 ILCS 5/4-401)
Sec. 4-401. When bank may charge customer’s account.
(a) A bank may charge against the account of a customer an item
that is properly payable from that account even though the charge
creates an overdraft. An item is properly payable if it is authorized by
the customer and is in accordance with any agreement between the
customer and bank.
(d) A bank that in good faith makes payment to a holder may charge
the indicated account of its customer according to:
(1) the original terms of the altered item; or…
[][][][]
(810 ILCS 5/4-406)
Sec. 4-406. Customer's duty to discover and report unauthorized
signature or alteration.
(a) A bank that sends or makes available to a customer a statement
of account showing payment of items for the account shall either return
or make available to the customer the items paid or provide information
in the statement of account sufficient to allow the customer to identify
the items paid. The statement of account provides sufficient
information if the item is described by item number, amount, and date of
payment.
(b) If the items are not returned to the customer, the person
retaining the items shall either retain the items or, if the items are
destroyed, maintain the capacity to furnish legible copies of the items
until the expiration of 7 years after receipt of the items. A customer
may request an item from the bank that paid the item, and that bank must
provide in a reasonable time either the item or, if the item has been
destroyed or is not otherwise obtainable, a legible copy of the item.
© If a bank sends or makes available a statement of account or
items pursuant to subsection (a), the customer must exercise reasonable
promptness in examining the statement or the items to determine whether
any payment was not authorized because of an alteration of an item or
because a purported signature by or on behalf of the customer was not
authorized. If, based on the statement or items provided, the customer
should reasonably have discovered the unauthorized payment, the customer
must promptly notify the bank of the relevant facts.
(d) If the bank proves that the customer failed, with respect to an
item, to comply with the duties imposed on the customer by subsection
©, the customer is precluded from asserting against the bank:
(1) the customer’s unauthorized signature or any alteration on
the item, if the bank also proves that it suffered a loss by reason
of the failure; and
(2) the customer’s unauthorized signature or alteration by the
same wrongdoer on any other item paid in good faith by the bank if
the payment was made before the bank received notice from the
customer of the unauthorized signature or alteration and after the
customer had been afforded a reasonable period of time, not
exceeding 30 days, in which to examine the item or statement of
account and notify the bank.
(e) If subsection (d) applies and the customer proves that the bank
failed to exercise ordinary care in paying the item and that the failure
substantially contributed to loss, the loss is allocated between the
customer precluded and the bank asserting the preclusion according to
the extent to which the failure of the customer to comply with
subsection © and the failure of the bank to exercise ordinary care
contributed to the loss. If the customer proves that the bank did not
pay the item in good faith, the preclusion under subsection (d) does not
apply.
(f) Without regard to care or lack of care of either the customer
or the bank, a customer who does not within one year after the statement
or items are made available to the customer (subsection (a)) discover
and report the customer’s unauthorized signature on or any alteration on
the item is precluded from asserting against the bank the unauthorized
signature or alteration. If there is a preclusion under this subsection,
the payor bank may not recover for breach of warranty under Section
4-208 with respect to the unauthorized signature or alteration to which
the preclusion applies.
(Source: P.A. 87-582; 87-1135.)