He noted, himself, that they might want to remove him so that he can't spread the knowledge any further, once he has taught the guild personnel the tricks. However, why would Yajin teach them anything if they don't pay? Maybe they intend to rob the money back from his corpse, but clearly they must agree on the terms on the surface, even if the end game is to kill him. The guild guys weren't born yesterday, so if Yajin offers to teach them for free, it's an alarm bell for them. If Yajin tries to negotiate for the highest price and a part of it paid beforehand, the guild people instead will relax.
something i suspect is mc might (try to) dilute the information in such a way that its ineffecent enough to not make it worthwile for them. he was coreced into teaching them he never said he would do it well.