A faqrah is a coherent unit of prose, and its fāṣilah is the final word of that unit. For sajʿ, one must identify the units and their terminal words, not merely adjacent words that sound alike.
For example:
“marfūʿah” — ending of the first unit.
“mawḍūʿah” — ending of the second unit.
These endings can then be compared for final sound and word pattern. Mistaking a coincidental internal rhyme for a fāṣilah leads to an incorrect classification.