Both are mechanisms used to determine the final purchase price in an acquisition, but they work differently:
Lock Box: The price is fixed based on accounts at an agreed historical date. From that date to completion, the seller cannot extract value except for permitted items. This gives certainty and avoids post-completion adjustments.
Completion Accounts: The price is adjusted after completion based on actual financials at the completion date, typically for working capital, cash, and debt-like items. This reflects the true position but can lead to post-deal negotiations.