BP Says They'll Pay. Why Not Get a Down Payment?

The BP oil spill in the Gulf of Mexico may end up being the biggest environmental catastrophe the United States has ever experienced (not withstanding the future impacts of climate change, of course). BP has said they will pay for all legitimate clean-up expenses.

I have a friend who works in the finance department at EPA's Energency Response Center. He told me that figuring out how to pay for the response so far has been a challenge.

I think President Obama should ask BP for a $500 million good faith check in advance, which could then be allocated more easily than trying to find funds within existing budgets. If BP is really intent on paying these costs, then why would they not agree to make a significant down payment now to show their good intentions? That would help the whole system work better.