I’d talk to the dealer about it for starters pointing out it was a demo and he agreed that it was ok to collect the straw from a distance away . If things go pear shaped pay what you think it’s worth ie the correct hire charge and deduct the price of the extra diesel and left in it . He can’t chase you for an unpaid bill then .
I’d invoice him £600 for getting the load of straw back then you can chase him if he chases you.