You are expecting the producer to distinguish between consumers that would have bought grain instead and consumers that would never buy the grain no matter how low the price went. In reality producers just don't have any reliable way to determine that, so they naturally consider freely given away produce to be a potential lost sale, or at least having a tendency to depress prices even if through indirect effects. And in fact they are almost always quite right to do so.
Well in this case the market is exhausted at the point of delivery. Which is to say the farmer has all his grain in wagons and he can drop it off in the grain elevators (full) drive somewhere else (always an option but raises his costs) or dump it. So perhaps we disguise the economic transaction by making our hopper look like an excess grain dump station :-).
Now I'm being humorous there but there might be some social engineering needed. And I wonder if one could promise a share of any future profit (if there is any) which comes from the use of the grain.