A tax lawyer writes:

In relation to your query:

You are a US taxpayer if you spend at least 183 days per annum in the U.S.
regardless of your place of domicile. The date of the payment is not,
generally speaking, relevant. The key factor is the days spent in the U.S.
during the fiscal year on which the income was earned. If LM was living in
the US and spends more than 183 days during the fiscal period in that
jurisdiction, then he’d be liable for US federal income tax in that country,
full stop.

A similar rule applies in Australia. If he cares about avoiding Australian
tax rates, he will probably ensure that he spends less than the minimum
required time in our jurisdiction. It would seem a sensible thing in the
circumstances.

Regards

Ezequiel T