Tell us exactly what it has to do, in terms of time, voltage, temperature, power, etc. Tell us what exact tests to which it can be subjected it has to meet in order to warrant payment in full. Tell us what the user interface must be. Tell us what the environmental conditions must be. Tell us what the Mean Time Between Failures must be. Tell us how it has to be packaged. Tell us when it has to be completed.

Tell us EXACTLY what you're willing to pay and EXACTLY when, and in what form payment will be presented.

Once those are known, one can work out an agreement.

RE