Fire clear_statement in a before_save hook

Tests pass, but I'm watching `#update_account` get closer to `#update_quantity`. I added a test earlier to capture the existing behavior when assigning prices inside `#update_account` fails (the account is changed, but the costs are not) but `#update_quantity` has a similar issue (quantity may update but but costs might not).
This feels a little strange. Does `reassign_price` apply a `save`? Wouldn't the statement get cleared after a save upstream?