Milestone payment
A milestone payment is an instalment of an off-plan purchase price that becomes due when a defined construction stage is reached: foundation complete, structure complete, and so on. The contract defines both the stage and the amount.
Also called: stage payment, progress payment, instalment
The whole idea is that money follows work. You pay for the foundation once the foundation exists, which protects you from funding a building that never gets built and protects the developer from financing your unit out of their own pocket.
It works only if the stage definition is precise and if someone checks it. Contracts vary enormously here. A clause reading "structure complete" can mean the frame is poured to roof level, or it can mean that plus the roof, plus the external walls, depending on what the annex says. The gap between the strictest and loosest reading of the same phrase is often several months of construction.
In practice the developer decides the stage has been reached and issues the invoice. Nobody independent looks at the building. The buyer, typically in another country, sees a document asserting that a stage is complete and a payment deadline attached to it, and pays, because refusing carries penalties and because there is no basis on which to refuse.
This is the specific point at which an off-plan buyer has leverage and does not use it. Before the payment is made, the contract is the buyer's instrument: a stage that has demonstrably not been reached does not trigger a payment. After the payment is made, the question becomes a complaint.
When an invoice arrives, the question is not whether the site looks busy. It is whether the specific clause that triggers this payment has been satisfied, and that is checkable.