Well, I originally answered as though a "burger flipper" was going to wander in for $50 or so in change, not $50M.
You still have the "was it authorized" issue, but a mitigating factor may be that you can prove delivery. I would hope that such large orders are being delivered by armored car. You prepare the order (shipment) in double custody and seal the "bag". It is then signed for when delivered. If you can prove they got the $, then the lack of authorizing signature is not such a big issue.
Don't you have a armored car services agreement with the customer? That could detail the commecially reasonable procedures authorizing the change order. Since it is not like you are sending the order to someone other than the customer, reasonable standards would be much less than sending out a wire transfer.
Hope that helps.
Al
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Al Miller, CRCM
Opinions expressed are my own and not necessarily shared by my employer.