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Record the contract, its the government procurement payment rules payment regime and the dates it turns on, so every later step reads the same terms instead of somebody's memory of them.

理由: A payment regime that lives in a PDF nobody opens is a regime that gets applied differently by each person who applies it. Registered once, the deadlines below are computed rather than argued about.

Record what was executed against each BoQ item for the period as a percentage of its contract quantity. The earned quantity follows from that percentage and the design quantity, and the earned amount from the same percentage and the position total, so one honest number per line produces the whole valuation.

理由: An interim payment certificate built from a single project percentage cannot be checked and therefore cannot be approved without a conversation. Measured line by line, the disagreement is about one BoQ item rather than about the whole month, and the rest of the money moves while that one is settled.

Read the valuation back against the bill and check the rates it used are the contract rates. Where a BoQ item has been measured past the quantity the contract carries, deal with it as a change to the position with an order behind it rather than as a larger figure on the same line.

理由: A variation order certified quietly is one the client can refuse at the final account, months after the work was built and paid for down the chain. Raising it as a change while it is small is the only version of that conversation where you are not asking to be paid for work already done.

Take the cumulative valuation, subtract what has already been certified, and apply retention at the contract percentage. The figure that leaves this step is the one the invoice is raised for, and the backup behind it is the measure that produced it. The advance is recovered on the proportion the period earned and the guarantee is reduced as it is recovered, so the two figures are read together rather than tracked in separate places by separate people.

理由: Interim payments are cumulative, so the month's money is a difference between two totals rather than a total of its own. Computing it from the cumulative figure is what stops a line being paid twice or dropped entirely when a period is re-measured.

Route the interim payment certificate to the Engineer, and to whoever the contract adds after them. Keep the approval date, because the invoice follows the approval and the payment clock runs from there, so an approval that slips moves everything behind it.

理由: Approval sitting in an inbox is the most common reason a payment is late, and it is invisible while it is happening because nobody has refused anything. A route with a date on each hop turns that into a question with an owner instead of a monthly complaint.

Raise the invoice against the approved interim payment certificate, carrying retention and any deduction the contract provides for, and reference the interim payment certificate on it.

理由: A demand for a figure nobody approved is one that will be returned, and the clock does not start on a returned document. Matching it to the interim payment certificate also means the accounts and the valuation tell the same story at year end without anybody reconciling them by hand.

Open a clock over the invoice starting from the day the invoice is submitted against the issued certificate, under the government procurement payment rules. Payment falls due 30 days later, and the clock records that date rather than leaving it to be worked out when somebody asks.

理由: The single most expensive mistake in interim payment is starting the count on the wrong event, because every deadline behind it inherits the error and none of them look wrong. Anchoring on the day the invoice is submitted against the issued certificate once makes the whole chain checkable. The window runs from a compliant submission, so an invoice returned for a missing attachment has not started anything even though it left your office. Recording what was submitted with it is what makes that visible on the day rather than at the end of the month.