When you are hiring a freelancer or agency, and writing your proposal there will be an option to choose an hourly rate or fixed rate.
You set the price for a project done at proposal writing time, these rates can be negotiated while interviewing the client. These will be given to freelancers on weekly basis.
There will be software, tracking freelancer activity continuously. You can see how many hours a freelancer has worked in a week. This tracking is 100% secure.
The payment will be released every week, according to hours worked in a week.