Mishnah Berurah — משנה ברורה

And if it tore etc. - Meaning, that it tore completely from the hole until the edge. Therefore, it is questionable; for even if the garment was wool and one sew's it with threads of other materials - that according to Rashi it is certainly permitted to do so; nevertheless, it is possible that according to Rav Amram it is invalid, because since none of the garment remained untorn [from the place of the tear till the edge], behold it was explained in the previous se'if that according to the first opinion, it's considered as if the corner was completely severed, and sewing it back up won't help, even if he wants to affix ציצית onto it afterwards. And it's possible that our case here is better, since the corner is whole from both sides, and it only tore in the middle, and that is the Shulchan Aruch's doubt. And see in the Ta"z who argues on this and rules that there is no doubt here, for everyone agrees that if you affixed ציצית after sewing it, that it is valid. Nevertheless, it is better to see to it that before sewing it, one places a piece of cloth that's 3x3 [fingerbreadths] under the corner and [therefore] will fulfill all opinions. And if that's not possible, one may be lenient like the Ta"z. For even without that, we wrote in se'if katan 15 that the opinion of the later rabbinic authorities is that sewing is considered an attachment and it works even when the corner tore completely from it's place.