```python
def line_tag(body):
    if not isinstance(body, str):
        raise ValueError("line_tag expects a string")
    return format(sum(ord(ch) for ch in body) % 256, "02x")


def check_line(line: str) -> str:
    if not isinstance(line, str):
        raise ValueError("check_line expects a string")
    if len(line) < 3:
        raise ValueError("line must be at least three characters long")
    if line[-3] != '~':
        raise ValueError("third-from-last character must be a tilde")
    
    body = line[:-3]
    expected_tag = line_tag(body)
    actual_tag = line[-2:]
    
    if expected_tag != actual_tag:
        raise ValueError("tag does not match the body")
    
    return body
```