I do worry about this but also it's about how much the extension if for. If we pay top FA money to extend him, and why would he take less, then I'd just sign him, when he does become a FA.
If you think we can extend him for less than the top offer will be, then okay, but where is that written into the trade? (Hint: it's not.)
This reminds me of the Kimbrell trade, which I was against, because I felt ewe could just sign a great closer to top dollar. Why trade so many pieces for a guy making top dollar? I ended up being wrong, because the salaries for top closers rose exponentially, immediately after the trade. He was underpaid soon after the trade.