Contract for development with piece-rate payment. How?

Question

Individual entrepreneur

Good afternoon.
Registered IP (with USN).
Now you need to conclude an agreement with the client for small work on the development and support of software.
We want to verbally (by phone, skype, etc.) stipulate tasks (including assessment and deadlines) and not fix them in the stage (attachment to the contract), so as not to waste time.
And from time to time draw up some kind of act (of completed work?) In which you indicate, for example, the number of hours spent, the cost of an hour and the total amount (preferably without describing the tasks themselves, just development).
Is this possible?
Will it raise questions?
Maybe someone has an example of such or similar contract and act?
One of the main questions is how to register responsibility?

Answer 1

In the contract, you prescribe:
1. The contractor performs work on the basis of a monthly fee (regular work performed by you every reporting period).
2. At the end of each reporting period, the contractor issues an invoice for additional work under the current contract. The fact of payment for the work confirms that the customer has no claims against the contractor in terms of quality, quantity of work performed.
3. In the invoice for payment of work, write simply point by point what you did. Or just additional work under the contract No./No.
Your lawyer will advise you on specific terms.
PS Yes, I'm not an entrepreneur or a lawyer :) So at your own peril and risk.