Reference for Sotah 52:3

מקנין על ידו פשיטא מהו דתימא (במדבר ה, יג) ושכב איש אותה שכבת זרע אמר רחמנא והא לאו בר הכי הוא קמ"ל

— but it only applies when she becomes prohibited [to the paramour] by this act of adultery; but where she was in any event forbidden to him, conclude that she is not [barred from marrying him]. Therefore he informs us [that she has to undergo the ordeal although the paramour was forbidden to her in any case and if guilty she cannot marry her paramour].

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