It is incumbent on the sales rep to understand and communicate what the products can accomplish.
If you are out over your skis on promises, you have to live with this loss. My best relationships are being honest and tell the customer that this and that are not ready yet.
As far as paying for the product or services, if the contract was understood there would be a small window to cure the problem(this is where you can build a great relationship with customer)or bring in your Director of Product to discuss. Customers who have made the decision to go with your company do not want to rip it out after 30 days.
Tough to answer, some sales take 60 days and some 18 months so. Shorter sales cycles the possibility of let down could be greater. Companies today are analyzing the heck out of product services and promises. If you know the product and know the customer it usually doesn’t get this bad...