The terms of this contract stipulate that the three, separate payments are all conditional on the system being operational by a specific date. If all parties have given assent to their contractual obligations, then David's subsequent request of additional remuneration in order to ensure fulfilment of the terms, is not covered by the contract, and as such, wholly unwarranted. Obiter dicta, any concurrent refusal to complete the project on-time, whether or not arising from the employer's own refusal to comply with demands that are, nonetheless, patently unreasonable, would have been tantamount to extortion; and, subject to the contract, the three would incur the stated financial penalty for late completion. David could not justifiably claim monetary compensation on an ostensible inability to deliver on the terms to which he previously assented: mitigating circumstances notwithstanding, Amber's sole refusal to facilitate such a claim would not entitle David to a legal remedy, under the conditions of his contract. However, it occurs to me that, in stating her assent to David's proposition (however unreasonable), she has entered into a verbal contract whose terms David has fulfilled, and he would thus have a legal case regarding her subsequent non-compliance. Furthermore, Amber is bound by the terms of her individual promises to Sharon and Ron, whose respective conditions were also fulfilled; and, as such, they too are equally entitled to make a claim against her.
That's how I've broken it down, anyway. You'll probably want to cite relevant precedents to reinforce your judgement.