As a general rule, money paid pursuant to a settlement agreement is gone for good. The bar for duress is pretty high--specifically, it usually (always?) requires a threat of bodily harm.
And, even if you could somehow prove a settlement agreement to be unenforceable, what would be the point? For one thing, you'd cancel the release of liability you obtained. And secondly, you already performed your part of the agreement (paying the money). In general, I wouldn't see any benefit in this course of action.
This is right, I don't think duress is relevant here.
As a hypothetical, if the company hadn't already paid the $5k, just agreed to do so in the future, they could have some legal arguments for not actually paying after the round closed. See http://www.lawnix.com/cases/alaska-packers-domenico.html.
(Of course, that's talking about whether or not there's any tenuously-supportable legal argument. Back in the real world, they should obviously just pay him.)
And, even if you could somehow prove a settlement agreement to be unenforceable, what would be the point? For one thing, you'd cancel the release of liability you obtained. And secondly, you already performed your part of the agreement (paying the money). In general, I wouldn't see any benefit in this course of action.