Midrash for Chullin 267:22
מתנות כהונה אין נגזלות:
<big><b>MISHNAH: </b></big>IF A PROSELYTE HAD A COW AND HE SLAUGHTERED IT BEFORE HE BECAME A PROSELYTE, HE IS EXEMPT FROM GIVING THE PRIESTLY DUES; IF [HE SLAUGHTERED IT] AFTER HE BECAME A PROSELYTE, HE IS LIABLE; IF THERE WAS A DOUBT ABOUT IT, HE IS EXEMPT, FOR THE BURDEN OF PROOF LIES UPON THE CLAIMANT.<span class="x" onmousemove="('comment',' In this case it would be upon the priest to show that the animal was slaughtered after the owner was converted to the Jewish faith.');"><sup>17</sup></span>
Sifrei Devarim
"from the slaughterers of the slaughtering": The time of the slaughtering is the criterion. From here they ruled: If a stranger converted and he had a cow — If it were slaughtered before he converted, he is exempt (from priestly gifts); if after he converted, he is liable. If the matter is in doubt (safek), he is exempt, and "the burden of the proof is upon him who would remove (the object) from his neighbor's domain.
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