I haven't seen anyone mention this, but the guy doing the email is almost certainly paid (or at least seen as successful) when he organises a face-to-face meeting. There's a fundamental imbalance of incentives here: the Sales Guy wants to get paid for taking time to sit down and negotiate, and the Programmer wants to get paid for running his business.

I think the major inefficiency is here: "My time is very limited, maybe you could first tell me roughly what this would be about?"

I would personally ask if they're most interested in features/option/pricepoint A,B or C. If it's not clear on the context I would ask a direct question for the three topics he would like to talk about.

Anything that steers the conversation towards what you want: whether this partnership is something you're interested in. At least this ways there's actually some motivation for him to actually tell you what's in it for your business.

[edit] the end was cut off for some reason