At some point in 1238, Lucas seeks out Roberto, whether at his lighthouse or during a visit to Arans, or, if Roberto prefers, invites him to dinner at his estate in Parma. Once pleasantries are done, Lucas gets down to the reason for talking to Roberto.
"I've been giving this matter some thought," he begins, "and I think I've reached a reasonable solution. I know you're interested in how I can boost spells with vis, just as I'm interested in how you can cast spells without exerting as much energy. I can't teach you the secrets of how I do it in detail. That would both be a violation of certain oaths, and take far too long. But the people of my bloodline have figured out a few shortcuts that other magi can take that allow this ability to be applied to a particular spell that has been studied sufficiently well to be mastered. It's more of a one-off thing that you'd have to apply individually to each spell. We've developed this halfway solution as something that we can share with other magi under certain circumstances, circumstances that I believe apply between us."
"So, if you like, I can teach spell-by-spell way to boost your magic in return for you teaching me how to cast spells using less energy. I'd have to ask you to promise not to teach the ability to anyone else without my permission, but I'd make the same promise regarding the secrets you taught me."
"I'd also be willing to make similar trades for the ability to harness or tether spells, if you'd like."
Basically, Lucas is offering to trade the Boosted Spell Mastery for the Stalwart Spell Mastery.