"from an accounting point i think this would be neutral imo. the...

"from an accounting point i think this would be neutral imo. the $20M would appear as an asset when we received it but there would be an equivalent liability as we owe $20M in iron ore shipments to repay this advance. of course, we'd be entitled to earn interest on this money if we still had it sitting in our bank accounts"

Well thats a moot point - if its an asset on the books at $20m its also a deferred liability if the cost of clean up is $20m ( or more ) so it means that $20m less profit wont be coming into the coffers as that amount of ore will be supplied for free.