Challah 3:5

If a non-Jew gave a Jew flour to make dough for him, it is exempt from challah. If he gave it to the Jew as a present before it was rolled, it is obligated in challah; after it was rolled, it is exempt. If a Jew and a non-Jew make dough together, if the Jew’s portion is not of a volume sufficient to obligate it in challah, it is exempt.

Challah 3:6

If a non-Jew converted to Judaism and he had dough, if it was made before he converted, it is exempt from challah; after he converted, it is obligated. In a case of doubt, it is obligated in challah. If a non-kohein ate this doubtful challah, he need not pay the extra fifth. Rabbi Akiva disagrees as to when dough becomes obligated in challah; rather than when it is rolled, he says that dough becomes obligated in challah upon baking, when the crust forms.