In my contract, users send along ETH to cover the costs of their Oraclize query.
Right now, they must send a fixed sum set by the contract owner. Whatever remains stays with the contract.
How can you ensure that they send just enough to cover the Oraclize fee + gas?
I'm not interested in reimbursing them after the callback. Rather, I'd like to have this at the top of the payable Oraclize function users call: