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    I think this is the correct answer because it addresses the core of the issue. Whilst the other answers provide useful suggestions, they don't fully tackle the problem. Your solution ensures consistency AND still leaves room for flexibility; Bob can still edit his quote as many times as he wants. I will implement @Hans-Martin's suggestion of invalidating quotes when Alice changes her requirements too. Commented Jan 22, 2020 at 13:07
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    One way how some card payment protocols do it is by having the token include (or be) a cryptographically secure hash or signature of the key fields of the transaction, obviously including the amount, so that at both sides (and any intermediary systems) it is possible to check if it matches, and nobody can maliciously alter the quote corresponding to the token without invalidating it. Commented Jan 22, 2020 at 19:43
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    Token is a good idea if you want Alice to be able to purchase at the old price for a certain period even if Bob changes the price. If that's not a requirement, Alice can just send the price she agreed on. If it doesn't match the price Bob is currently quoting, you can return a failure code with the updated price so you can display a message about the price increasing... Commented Jan 23, 2020 at 2:01
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    This makes sense, you shouldn't think of it as Bob changing a property of the quote - a quote should be immutable. Rather by changing the price Bob is withdrawing the original quote and creating a new one. Generally in systems I've seen this, the id is a 2-tuple - the first element is fixed when Bob creates the quote and the second increments each time Bob amends a field. That way you can track change over time but ensure that both parties are manipulating the same object. Commented Jan 23, 2020 at 3:05
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    Not just shouldn't you change the quote mid-transaction because it makes Bob look bad to Alice, it's probably (depending on jurisdiction) illegal to do so. I know that here consumer protection law specifically says you cannot do that. Especially you can't just charge someone's bank account or credit card more than the agreed upon amount. You can charge less, but if you do so you're then not allowed to later charge the rest as well. Commented Jan 23, 2020 at 6:14