How does the Outrider work in Agricola?
The Agricola Compendium is a superb resource for this kind of rules question. In this case, it has your answer:


  You receive the grain before you've taken the action. For example, if you use a sowing action, you can immediately sow the grain from the Outrider.


It sounds like you're interested in the principle behind the answer. I'd love to explain it, but I can't. The Agricola rules are generally very good about consistency of terminology - if something says "an oven" it means any of the cards with "Oven" in the name and no others. If something says "receive wood on an action space" (Pieceworker), that's different to "take wood that is on an action space" (Mushroom Collector). But sadly, one area where they're not so consistent is precise details of timing like this.

I'd love to be able to say things like in Magic: the Gathering, something like "Some cards have the pattern "Whenever [player] [takes action], [effect]." In this case, the [effect] happens at the start of the [action]." But that doesn't seem consistent with the rules on cards like Compulsory Education, Merchant. And it's unclear whether if the Wood Distributor has put some wood on a Major Improvement space, and you take it with a Clay Worker to get an additional clay, do you get the clay in time? I don't think Agricola has specific timing rules to allow you to state the answer with complete confidence in cases like this.