Responsa for Yevamot 201:8
<br><br><big><strong>הדרן עלך נושאין על האנוסה</strong></big><br><br>
HER <i>HALIZAH</i> IS VALID,<span class="x" onmousemove="('comment',' Though the shoe required for halizah purposes should properly be a sandal made of hard leather and consisting of a sole with straps attached for fastening it to the foot. ');"><sup>24</sup></span> [BUT IF] WITH A SOCK<span class="x" onmousemove="('comment',' [H] Cf. infilia, [G], shoes or socks made of felt. ');"><sup>25</sup></span> IT IS INVALID; IF WITH A SANDAL<span class="x" onmousemove="('comment',' [H], [G]. ');"><sup>26</sup></span> TO WHICH A HEEL IS ATTACHED IT IS VALID, BUT [IF WITH ONE] THAT HAS NO HEEL IT IS INVALID. [IF THE SHOE WAS WORN]<span class="x" onmousemove="('comment',' Cf. Rashi. According to others the law refers not to the shoe itself but to the sandal straps. ');"><sup>27</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.