```python
from typing import Dict, Optional

def fresh_value(entry: Dict[str, int], now: int) -> str:
    if not isinstance(entry, dict) or len(entry) != 3:
        raise ValueError("Invalid entry format")
    if not isinstance(entry['value'], str):
        raise ValueError("Invalid value type")
    if not isinstance(entry['stored'], int) or entry['stored'] < 0:
        raise ValueError("Invalid stored value")
    if not isinstance(entry['ttl'], int) or entry['ttl'] <= 0:
        raise ValueError("Invalid ttl value")
    if not isinstance(now, int) or now < entry['stored']:
        raise ValueError("Invalid now value")
    if now >= entry['stored'] + entry['ttl']:
        raise ValueError("Record no longer usable at now")
    return entry['value']
```