Reference for Chullin 88:14
שלא אכלתי מבהמה שלא הורמו מתנותיה
And it has been taught: A judge who decided an issue declaring the one party entitled to a thing and the other disentitled, or who pronounced aught to be unclean or clean, or forbidden or permissible, likewise witnesses who gave evidence in a law suit, these may [in law] buy the matter that was in dispute, but the Sages have said: 'Keep aloof from anything hideous or from whatever seems hideous'! - This applies only to matters which are bought by appraisement;<span class="x" onmousemove="('comment',' I.e., such things as are bought and sold by a general estimation of their weight or an approximate assessment of their value without resorting to the usual practice of weighing or measuring. Only in such a case is there ground for suspicion.');"><sup>8</sup></span>