I think the optimal sequence is:
- Mine Yasuji for information. Act concerned about what he's revealed, and interested in his offer. Don't commit to anything, citing the need to seek the Hokage's approval, but have him walk away thinking he's won.
- Discuss the information we've gathered, what we're willing to offer him, etc. Put together a plan and get Asuma's approval on it.
- Have the second meeting, and do a one-eighty: immediately reject the seals-related part of his offer, act only marginally interested in the other parts.
- Seals: In reality, we don't care about it because 1) the problem is years away, and by the time it's a problem we'll have become a jounin-level biosealmaster capable of fixing it on our own, 2) depending on how the seals' expiration function is made, we could likely steal a refreshed design from another Wakahisa, even from Yasuji here right now. (Not that we can say any of that, since the first is effectively bloodline theft and the second is an explicit threat of hostile action.) I'm confident we don't need external assistance there.
- Ami: Just claim we're confident she can save herself, or that Aunt Ren owes us a favour. Get Keiko in on it if possible, for maximum impact.
- Koi: Claim that we could just breed our own specialized chakra beasts, or something.
Ideally, we'd handle him like we've handled Kabuto, switch to dismissing his offers and pressuring him right now. But it's a significantly more complicated situation that we also know less about, we need to take a break do discuss it, so trying to put him off-balance by a mood whiplash is probably our best bet. (If we want to maximize what we get out of
this deal, that is. If we want to establish a long-term trading relationship with the Wakahisa, it might make sense to make them happy about the deal and happy about the easily-manipulated Yasuji, so that he stays the ambassador whom we can pressure into accepting an even more lopsided deal at some future point. I'm inclined towards not doing that.)
Edit: Ohh, we can also question his ability to deliver on his promises by pointing out that we'd be trading bloodline secrets and he's nowhere near a Clan Head. Seems like it'd work better for fishing for information, though, so we should probably do it during Phase 1 and subtly.