How to negotiate when professor asks for your help with a grant A professor who I collaborate with on a project (not my advisor, he's not even from my university) asks me to help him to write a major grant proposal that is due in a week.

In an initial phone conversation with him, he said it would be great if I could help him for around four hours every day. I asked him whether (if I did everything he wanted) I could be hired as a postdoctoral scholar if he got the grant, and then asked about the salary and duration of the postdoc (just to make sure there wasn't a mismatch of expectations, because a two year postdoc is much more comfortable than a one year postdoc, etc). We eventually settled on him paying me at an hourly basis, and with no guarantee that I'll get funded even he gets the grant. We were both very happy with this arrangement, though I think we both felt that in the future, it would be better to come to an agreement faster so that more work can be done (we spent half an hour reaching an agreement).

If this situation happens in the future, how can I balance the need to avoid a mismatch of expectations with the need of providing as much help to him as possible?
You negotiated for 30 minutes to agree upon a 20 hour contract (four hours per day, for one week). That's an overhead of 2.5%. I doubt you can do much better...