Answer:
An excerpt from 31 CFR 210.10(a): "An RDFI shall return any benefit payments received after the RDFI becomes aware of the death or legal incapacity of a recipient or the death of a beneficiary, regardless of the manner in which the RDFI discovers such information."
I doubt you knew of your depositor's demise before the SSA deposit was received for credit to his account. You should ensure that the September 8 payment is returned (but it's not likely to show up).
His September 8 payment was earned for the month of August, so even if you were to send it back, his survivors would get it back (eventually).
First published on BankersOnline.com 9/17/12