Great idea and all the freelancer website alike now applying such feature.
The Milestone should be updated to keep pace with other competitors ASAP 🙂
I mean let’s say a project has 3 milestones and each milestone costs 100 USD and total of project is 300 USD. After each milestone is resolved the freelancer should get the payment for that milestone. After the 1st milestone is finished he should be able to get 100 USD. Almost all freelancer websites including upwork support this feature.