Responsa for Yevamot 201:9
מתני׳ <big><strong>מצות חליצה</strong></big> בשלשה דיינין ואפילו שלשתן הדיוטות חלצה במנעל חליצתה כשרה באנפיליא חליצתה פסולה בסנדל שיש לו עקב כשר ושאין לו עקב פסול
BELOW THE KNEE<span class="x" onmousemove="('comment',' Where, for instance, the levir (according to Rashi) had his foot amputated. According to the other interpretation 'below', and 'above' the knee refers to the position of the straps on the leg. ');"><sup>28</sup></span> THE <i>HALIZAH</i> IS VALID, BUT IF ABOVE THE KNEE<span class="x" onmousemove="('comment',' Where, for instance, the levir (according to Rashi) had his foot amputated. According to the other interpretation 'below', and 'above' the knee refers to the position of the straps on the leg. ');"><sup>28</sup></span> IT IS INVALID. IF THE WOMAN PERFORMED THE <i>HALIZAH</i> WITH A SANDAL<span class="x" onmousemove="('comment',' [H], [G]. ');"><sup>26</sup></span> THAT DID NOT BELONG TO HIM,<span class="x" onmousemove="('comment',' The levir. ');"><sup>29</sup></span>
Teshuvot Maharam
A. Bediabad, in this case, means after the rite was performed. Nevertheless, the aforementioned performance of the rite is invalid, for a different reason: the shoe used was sewed with flaxen thread and, therefore, was never fit for this purpose. Since, however, the levir performed an invalid rite of halitzah, neither he nor his brothers are now permitted to marry the widow. Halitzah by the same levir being the only manner of releasing her, he may be forced, by persuasion or by flagellation, to repeat the rite with the proper shoe. For now the Mitzvah revolved on him alone, and we are permitted to scourge a Jew until he perform the required Mitzvot.
This Responsum is addressed to Rabbi Menahem of Würzburg.
SOURCES: Am I, 93, 94.