I feel that Yajin just straight up declining the offer was a mistake; he should have listed his concerns and then guided the conversation towards an outcome that was acceptable for him with the knowledge that the Physician's guild wants the knowledge on how to gather the famour herb. Instead, by declining, the ball was sent right back to the guildmaster's camp who could proceed with another demand while implying that refusal for that would not be acceptable. Even then, he could have accepted while setting a condition that the adventurer's guild should be involved for these negotiations (through a request to the adventurer's guild or something). There weren't any details on what form gratitude from the physician's guild would entail.
Yajin understands the importance of intel for an adventurer, so he really should have been trying to ask about how they had learned that he was the one who had delivered the famour herb, and then tried to guide the conversation towards something like having a deal made between the adventurer's guild and the physician's guild so that, if a deal is reached, it will have the approval of both guilds and he won't have to worry about reprisal from either side. Though there is always the danger that if a deal falls through (if the adventurer's guild feels that they have a good hand, they might be too greedy) then he'll still be in danger, but since the guildmaster offered to poach him, I think Yajin had the necessary leverage to make both guilds reach a compromise.
Yajin mentioned the adventurer's guild wouldn't take it lightly for him to get poached by the physician's guild, but isn't there the same issue with an adventurer's knowledge being poached? Maybe the adventurer's guild has a more hands-off approach to their guildmembers in terms of getting their knowledge?
Well, we'll see next chapter who these two guys are, but there sure are a lot of people interested in Yajin. He sure has trouble keeping a low profile. XD