The affix ṭhañ comes in the sense of matup after the words 1. śata and 2. sahasra when they follow after niṣka,
Thus निष्कशतमस्यास्ति = नैष्कशतिकः, नैष्कसहस्रिकः ॥ But not so after सुवर्णनिष्कशतं, it being never so found in usage.,
शत-सहस्र-अन्ताच् च निष्कात्
Adhyāya 5 · Pāda 2 · Rule 119