I have no expertise in any of this, but I was assuming it would be something along the lines of an IDIQ (indefinite delivery, indefinite quantity) contract vehicle: https://en.wikipedia.org/wiki/IDIQ
That makes some sense, though it seems they'd have to deliver something in that case. I'm sure one could keep the charade going for a little bit by delaying/delivering stuff that didn't work, but eventually the docs in the army medical corp would start to raise red flags and the jig would be up. One hopes anyway?