Assume that this "deal" is for some product or service that you are offering to a client. The 5 questions to ask up front are to clarify .....
1. Clear identification of the products, services, skills, materials and performance factors required to complete the project so that the resources and associated cost can be estimated accurately.
2. A description of the conditions under which the project personnel will be required to perform and any related environmental or location factors
3. Specific references to product specifications that govern an acceptable product or services performance outcome and delivery acceptance.
4. A schedule for the project that identifies discrete delivery dates for products and specific start and end dates for supporting labor.
5. A precise description of customer furnished material or facilities required and when it will be made available.
Another way to look at it is this ....
1) What are the details of the problem you are trying to solve?
2) What are your desired solution, schedule and cost goals?
3) What do you expect me to deliver?
4) What resources are available?
5) Can I realistically solve your problem, achieve your goals with the the available resources and meet your delivery expectations?
The fact that it is a new business relationship (or so implied by the question) means nothing in terms of defining the project other than being a bit more cautious of new relationship matters. You may want to have inchstones instead of milestones so that you will mitigate risks and get faster relationship feedback and response. You mayalso want to get some credit checks or references to gain confidence. Other than that, try to have an early delivery and payment so you can test the business relationship before you commit too much of yourself.