Very interesting. The stamp over the FRB side seems akin to righting 'both halfs' of the bill. There are those who suggest the organic treasury of the United States to have stopped existing in 1921. While I don't necessarily agree, in that event the powers would still vest in the People.
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Re: Gifts
If he is regarding it to be a gift, a gift would not be applied to pay taxes. It might be best to the bills to the USPS to get a money order and keep a record associating the (International?) Postal Money Order with lawful money or pay with the bills at a local revenue office that has a cashier.
Re: Serialization of Stamps
Using registered, numbered stamps would remove the need to send photos of the bills with the view that a list of the serial numbers and denominations would be sent to Treasury, District Court and/or State AG.
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