TT #673 · Parashas Ki Seitzei · Seventh Aliyah · Passuk 25:05
When brothers dwell together and one of them dies and has no son, the wife of the dead shall not be outside to a strange man; her husband’s brother shall come upon her and take her to himself as a wife and perform the duty of a husband’s brother for her.
Torah Temimah points to Yevamot 13a.
TT lemma: לֹא תִהְיֶה אֵשֶׁת הַמֵּת הַחוּצָה לְאִישׁ זָר.
הָא אִם יֵשׁ לוֹ בֵּן – מִיתָה מַתֶּרֶת.
English: This implies that if he has a son, death itself permits her (to remarry freely).
On the daf / in context: The restriction “not outside to a strange man” applies only in the absence of offspring.
Implication from the negative formulation.
When the deceased leaves offspring, the widow is free to marry any man without chalitzah or yibbum.