I'm going off memory here, so I might be wrong: if you don't have enough room in your inventory, the grain might be on the ground, or the game won't give it to you until you actually have the space (most likely the first). If I remember correctly, the grain occupied the same room as an armor (4 squares of the inventory).
nullSong: I also have a problem with this quest. I was supposed to give a tuning fork to a girl in order to get pants? But my tuniforks have disappeared? Did the prank stone steal my tuning forks? Where can I find more of them?
Prank's Stone doesn't make tuning forks disappear.
Doing a quick search: You can get at least one from spoils after combat (with Nighthawks in a particular location), and another from a quest (opening a rusty barn door), the ones that want the fork point you towards this quest.
There is also the possibility of buying one, but only in chapter 4
