وَإِنْ كَثُرَ expresses concession: “even if it becomes abundant.” The continuation, فَإِنَّ عَاقِبَتَهُ تَصِيرُ إِلَى قُلٍّ, states the contrasting outcome. Understanding the structure helps a learner see the contrast between temporary outward gain and the ultimate consequence.
كَثُرَ is a past-tense verb and تَصِيرُ a present-tense verb. The suffix ـهُ in عَاقِبَتَهُ refers to usury. إِلَى قُلٍّ expresses a movement toward diminution.
Recognizing this pattern also helps with similar Arabic structures that combine “even if” with an emphatic conclusion.