وَإِذَا أَصَابَتِ الْخُفَّ نَجَاسَةٌ لَهَا جِرْمٌ، فَجَفَّتْ، فَدَلَكَهُ بِالْأَرْضِ: جَازَتِ الصَّلَاةُ فِيهِ.
Translation: If impurity having a material body adheres to a leather khuff, then dries and is rubbed off against the earth, prayer in it is permissible.
Laha jirm means impurity with a discernible material mass. The conditions here are a solid residue, its drying, and rubbing that removes the mass.
This concise rule is not applied indiscriminately to wet impurity or material that has permeated the leather.