Reference for Pesachim 173:3
<big><strong>גמ׳</strong></big> שמעת מינה יש ברירה מאי רוצה בשעת שחיטה
HE WHO IS HALF SLAVE AND HALF FREE<span class="x" onmousemove="('comment',' E.g. 'he had belonged to two masters, and one had manumitted him.');"><sup>4</sup></span> MUST NOT EAT OF HIS MASTER'S.<span class="x" onmousemove="('comment',' As we assume that his master did not count in the free half.');"><sup>5</sup></span> <big><b>GEMARA: </b></big>[Hence] you may infer from this that selection is retrospective?<span class="x" onmousemove="('comment',' Lit., 'there is bererah'. Bererah is a technical term denoting that a choice or selection made now has retrospective validity in a legal sense. For it is assumed that the Mishnah means that the woman may eat of whichever offering she desires now, though she had not yet made her choice when it was killed and its blood was sprinkled. But the Passover-offering may be eaten only by those who had registered for it and on whose behalf it was killed. Hence when we say that her present choice permits her to eat thereof, it proves that this choice is retrospectively valid, as though she had declared it before the offering was killed. Actually there is a controversy (B.K. 51b; Bez. 38a; GIT. 25a) in this matter.');"><sup>6</sup></span>