Sales and Engineering have always had a coupled relationship. Enterprise sales requires tailoring solutions to high paying customers which evidently results in engineering requirements to be shifted. This is true for Product relationship with Sales. As new features are released the level of maturity of the feature may not be on par with the expectations and quality level of Sales. So what can you do?
Handling exceptions with Sales early on is very important. Explaining the product release concepts (prototype, alpha, beta, MVP, MMP) and setting a collaboration boundary of MMP will allow for process driven building by the product leader.
At the heart of the sales and engineering relationship is trust with the customer. As a Sales agent in a startup, your goal is to sell yourself first, build trust, then sell the problem, then sell the solution - and your product is an implementation of the solution (along with other competitors but yours is better).
Engineering builds trust with the customer by delivering what the customer needs to use.