The only way the yankees get that money is if the contract that is in place stays in place. Texas agreed to pay a certain percentage per yr of the existing contract at the time of the trade. An extension keeps that existing contract in place, it just adds yrs to it. So the yankees can still get the money from texas. Once he opts out and that contract is moot, the money from texas is forfeit. Hence, the yankees have an upper hand on anything anyone can give him, because they already have 46 mil paid by another team. Assuming you consider the fact that you will have to give him a set amount over a large number of yrs. So assume the yankees extend him for 3 yrs at 27 mil per, which is what he will make for the final 3 yrs of the deal in NY. That is 162 mil over the next 6 seasons. 46 of that will be paid by Texas within the first 3 yrs. So looking at it from a "what do we have to pay" scenario, the yankees can get ARod services over the next 6 yrs for 116 mil, which would come out to 19.3 mil per yr average out. Therefore, the yankees can offer the most without actually having to pay the most. Hence, the yanks should be able to give him the best deal.
Now if he hates NY and wants out, he likely wont get that kind of deal nor would he have the yankees bidding since the only way he opts out is if the yankees dont make a pre-emptive strike and essentially pull themselves out of the running. Allowing ARod to opt out and then signing him again is a bad business move and a waste of 46 million dollars.