English Translation
I don’t know how he made that money.
What an unnatural translation!
彼がどうやってそのお金を稼いだ（かせいだ）か分からない。= (Lit.) I don’t know how he earned/obtained the money.
彼の金の出処（かねのでどころ）は不明（ふめい）だ。= (Lit.) The source of his money is unknown.
We never say 金を「作る」. It can merely mean “to synthesize gold”, which sounds unnatural.