If you didn't have a document that says they will invest X amount under Y conditions, you did not have an offer. It is not unusual in business for people to make a conditional verbal indication of feedback, such as that you need to make more progress in X area before they will invest.

From the sounds of it, people are offering to invest up to $2.5M in a consumer company with no users based on a deck and MVP? You should take the money, that sounds generous. If someone backed out of closing a signed share purchase agreement, talk to a lawyer. This just sounds like a hard business lesson.