Sounds like it is lawyer time!
If in fact the dealer repaired it without providing you with a written estimate and receiving your specific approval, you may have a very strong case against the dealer.
You may not have to pay more than a "reasonable" inspection or diagnostics fee (or what ever you may have authorized them to do when you brought the tractor in to them).
While the break may well have been due to a latent manufacturing defect, the previous owner may have done something (unknown to you) that severly overstressed the unit, causing a crack(s) that just happened to completly fail while doing a seemingly normal light-duty task. Certainly have not heard of other similar occurances.
Good Luck,
Rip