The basic fiduciary premise is that an attorney-in-fact cannot perform any act from which he would derive personal benefit. So, you might reasonably assume that the attorney-in-fact is acting on behalf of the principal; i.e. the principal goes in section A and the attorney-in-fact goes in section B. If the attorney-in-fact acknowledged that he was cashing the check on his own behalf he would be admitting a breach of responsibility.
Pollyanna would pat me on the head for that analysis.
The larger issue here is that fiduciaries have to keep records and cash withdrawals are an anathema to record keeping; an attorney-in-fact should not be making cash withdrawals of any size. Plug this into your SAR consideration process.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.