What would be the net effect if the non-Jewish person to whom I have sold my chometz for Pesach converts to Judaism over Pesach?
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From various Mishnayot (see below) we see that a Ger's possessions seem to continue to belong to him after he converts.
In that case, the moment your buyer converts, he owns Chametz on Pessach - Chametz from which everybody is now forbidden to derive benefit from - similar to any Chametz a Jew owns on Pessach.
Result: He would not be allowed to sell it back; he would have a Mitzva to burn it ASAP.
We see that the crop is his; and his ownership carries over from before his conversion.
Here it's even clearer that the dough belongs to him even after his conversion.
Here again we see that his ownership of the cow continues through his conversion; freeing/obligating him with Matnot Kehuna (זְּרוֹעַ לְּחָיַיִם קֵּבָה ).