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.)
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.)
IANAL. TINLA.