```python
from typing import List, Tuple

def net_tally(sheets: List[str]) -> str:
    if not isinstance(sheets, list):
        raise ValueError("sheets must be a list")
    
    report = []
    total_sold = 0
    total_returned = 0
    
    for sheet in sheets:
        if not isinstance(sheet, str):
            raise ValueError("sheet must be a string")
        
        rows = sheet.split('\n')
        for row in rows:
            if not row.strip():
                continue
            
            fields = row.split('|')
            if len(fields) != 3:
                raise ValueError("row must have exactly three fields")
            
            item = fields[0].strip()
            sold = fields[1].strip()
            returned = fields[2].strip()
            
            if not sold.isdigit() or not returned.isdigit():
                raise ValueError("sold or returned field must be a string of decimal digits")
            
            if int(returned) > int(sold):
                raise ValueError("returned total cannot exceed sold total")
            
            total_sold += int(sold)
            total_returned += int(returned)
            report.append(f"{item:>{len(item)}}  {int(sold) - int(returned)}")
    
    if not report:
        return ""
    
    total = total_sold - total_returned
    report.append(f"{len(report):>{len(item)}}  {total}")
    
    return '\n'.join(report)
```