```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")
    if not all(isinstance(sheet, str) for sheet in sheets):
        raise ValueError("each sheet must be a string")
    if not all(sheet.strip() and sheet.count('\n') == 1 for sheet in sheets):
        raise ValueError("each sheet must be a single newline-separated row")
    if not all(sheet.count('|') == 3 for sheet in sheets):
        raise ValueError("each sheet must have exactly three fields")
    if not all(sheet.split('|')[0].strip() for sheet in sheets):
        raise ValueError("each item name must be non-empty")
    if not all(isinstance(sheet.split('|')[1], str) and isinstance(sheet.split('|')[2], str) for sheet in sheets):
        raise ValueError("each sold and returned field must be a string of decimal digits")
    if not all(sheet.split('|')[1].isdigit() and sheet.split('|')[2].isdigit() for sheet in sheets):
        raise ValueError("each sold and returned field must be a string of decimal digits")
    if not all(sheet.split('|')[1] <= sheet.split('|')[2] for sheet in sheets):
        raise ValueError("returned total must not exceed sold total")

    item_net_counts = {}
    item_total_sold = 0
    item_total_returned = 0

    for sheet in sheets:
        for row in sheet.split('\n'):
            if row.strip():
                item, sold, returned = row.split('|')
                item = item.strip()
                sold = sold.strip()
                returned = returned.strip()
                if sold.isdigit() and returned.isdigit():
                    item_net_counts[item] = int(sold) - int(returned)
                    item_total_sold += int(sold)
                    item_total_returned += int(returned)

    if not item_net_counts:
        return ""

    item_net_counts = sorted(item_net_counts.items(), key=lambda x: len(x[0]))
    item_net_counts = [f"{item:<{max(len(item) for item in item_net_counts)}} {item_net_counts[i][1]}" for i in range(len(item_net_counts))]
    item_net_counts.append(f"{sum(item_net_counts, (0, 0))[0]:<{max(len(item) for item in item_net_counts)}} {sum(item_net_counts, (0, 0))[1]}")

    return '\n'.join(item_net_counts)
```