Midrash for Chullin 267:15

"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.