In building contract disputes, the amount 'due' under a payment certificate is the amount payable on account of the contract price as stated in the payment statement, not the net amount remaining after a disputed set-off under a separate contractual provision. A legal set-off does not extinguish the underlying debt but merely provides a defence; the debt remains extant until judgment is recovered on the set-off. When construing terms of settlement, the court cannot look at alterations made during negotiations to determine the parties' intention.
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