I thought big companies like this were supposed to have smart lawyers.
While Modus is probably in the moral right, they are definitely in the legal wrong on this one. The proper way to withhold payment in cases like this is for Modus to have placed the amount for the payment in escrow until such time as Nimbus fixed the problems with equipment. The exact process for doing so may vary from place to place, and I expect it involves a judge giving his approval for placing the money is escrow.
I know someone who had a similar problem and that's what they did. Granted theirs was housing related (mushrooms growing in the basement carpet because of a leak problem the landlord refused to fix) but the same principles applied. The aggrieved party got back rent for the better part of the year plus interest when the lease expired.