I am sure there is legal recourse for the soldier. By agreeing to the contract there was an inherent risk.
Think about it this way: If a person signed a contract to perform a dangerous duty for a sum of money. In this case, it is front line battle where the risk of death is high. You could offer these people large sums of cash and reclaim it when their efforts to complete the contract causes them to not be able to fulfill the terms of the contract. The DoD could know full and well that many of the soldiers would be unable to fulfill the terms of the contract...It would be in their best interest to put the soldiers with the highest bonus in the most harm so they could reclaim the bait.
There are laws against lendingg money to those who cannot pay. What is the difference here?
I am sure there is legal precendent against such things