Bechorot 61 - One's Agent Should Only Help

We learned that if ninth or eleventh animals are mistakenly called "tenth," they become sanctified. However, this is only true when one tithes himself. If someone sent an agent to tithe his flock, and the agent miscounted, calling the ninth or the eleventh animal "tenth", then the ninth is indeed sanctified, but the eleventh is not - these are the words of Rav Pappi. Rav Pappa disagrees and says that neither the ninth nor the eleventh become sanctified, because the owner can tell the agent, "I sent you to benefit me, not to harm me, therefore your agency is void." And Rav Pappi? He says that when the agent called the ninth animal "tenth", his agency was still in power, he only lost it after this action, and tithe sanctified in error still counts.

But how is this different from one who sends an agent to separate the kohen's portion (terumah) from his grain? There, even if the agent miscalculated the generosity of the owner and gave 10% more, or 10% less, his actions stand? -- For kohen's portion, there are indeed people who give generously or stingily, but for animal tithe the owner can say, "You should not have erred."

Art: Joseph Augustus Knip - A Flock Of Sheep In A Stable