Rule 1: For the name-dependent classes identified by the book, indefinite use can restore full declension: طَلْحَةٌ، عُمَرٌ، أَحْمَدٌ when referring to another unspecified bearer of that name.
Rule 2: A diptote with al- or as the first member of iḍāfah takes genitive kasrah: بِأَحْمَدِكُمْ، بِالْمَسَاجِدِ.
These are distinct rules. Always determine the cause and the noun’s syntactic position.