erm, something similar happened to my colleague before. but the
amount was ALOT more than ur 400 usd.
anyways, if the person who key in the wrong amount realize it soon
enough and inform the bank, i think they can reverse the
transaction then transfer the money back to the original
acct.
i not sure but my colleague did mention that she check her account
the money alot then she didnt touch any bit of it then few days
later it was back to the original amount. 
so i guess you probably should just leave it there first or clarify
with the bank..
