If your state has adopted the standard Uniform Commercial Code...here is Section 3-407
ยง 3-407. ALTERATION.
(a) "Alteration" means (i) an unauthorized change in an instrument that purports to modify in any respect the obligation of a party, or (ii) an unauthorized addition of words or numbers or other change to an incomplete instrument relating to the obligation of a party.
(b) Except as provided in subsection (c), an alteration fraudulently made discharges a party whose obligation is affected by the alteration unless that party assents or is precluded from asserting the alteration. No other alteration discharges a party, and the instrument may be enforced according to its original terms.
(c) A payor bank or drawee paying a fraudulently altered instrument or a person taking it for value, in good faith and without notice of the alteration, may enforce rights with respect to the instrument (i) according to its original terms, or (ii) in the case of an incomplete instrument altered by unauthorized completion, according to its terms as completed.
Basically, if a teller accepts an altered check (whether it is the payee or dollar amount) for cash or deposit, the drawee bank can make a warrenty claim against your bank and you will have to try and collect from your customer.
There's no black and white to accepting or refusing an altered check. That is a risk based decision you have to make for your institution.
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