How does he know the wager very well?
Because Osric told him.
Is this an opportunity for Hamlet to mess with Osric some more? He’s clearly present – the king has just told him to give them the foils. I’m guessing that this line would have to reference Osric somehow – either Hamlet delivers part of it TO Osric or nods at Osric or touches him as he receives a foil. Something.