Agares had been keeping many powerful alchemists around for the sake of his equipment, so he definitely knew what Saleen was talking about. That was precisely the reason why he was so surprised—that piece of 12-dimensional metal was the best material that he knew about.
Being able to manipulate it to such a high level of precision indicated that there had been few errors during the crafting process.
Saleen threw his own piece of the 12-dimensional metal into his Starline Ring, but gave the other fragment to Agares instead.
Agares looked at Saleen, feeling puzzled. Saleen explained: "We'll have an exchange when I'm done crafting it. If I go back on my word, well, you'll still have the other half of the material, which would be enough for you to craft your ideal weapon, anyway."