In the case with Patrick, the number of cherries WAS specified. Well, the interest rate was, anyway. And Patrick failed to hold up to that interest rate.
That's certainly a breach, but not even the biggest one. The main problem here is that Patrick failed to repay the principal on the commonly agreed due date, not that he failed to repay interest as agreed.
So to use your cherries example: someone agreed to sell one truck with 5k lbs of cherries to be delivered at a specified location at a specified time, and someone agreed to buy same. The buyer paid the seller on the spot. At time of delivery, seller informs buyer that a) there's only 4500 lbs of cherries in the truck and b) the truck is "somewhere in Argentina" but will be making it towards the specified location "as best it can".
Well duh.