Can a document obtained from a place that deals in usury—despite its price being paid later—affect the legitimacy of the work acquired through it?

The devil's whisper has afflicted you, and your work is not forbidden because of this document, whether you paid for it or not. This is because the sin of usury is tied to the borrower's liability, not to the money itself. You must seek refuge with Allah from the accursed devil, ignore this whisper, and master your work.