YC's answer to this is "If you verbally commit to an investment with a YC company they will memorialize it instantly with an SMS or email, which they will ask you to confirm substantially instantly. If you renege on a deal committed to by this protocol, you will be entered into a list of Unserious People. You very much don't want to be Unserious People to all future deals associated with YC companies."
That's for actual commitments. Investors historically have a very diverse vocabulary of ways to phrase "No" (for example, any sentence which begins with "I will invest if" means "No, but I like option value, particularly when it is free.") and entrepreneurs historically have a reality distortion field where they hear "No" and think "I didn't hear Never so that's practically a Yes!"
That's for actual commitments. Investors historically have a very diverse vocabulary of ways to phrase "No" (for example, any sentence which begins with "I will invest if" means "No, but I like option value, particularly when it is free.") and entrepreneurs historically have a reality distortion field where they hear "No" and think "I didn't hear Never so that's practically a Yes!"