Let's say I have found a site where you can buy some products. The site follows a typical shop flow, e.g. a shopping cart and a checkout.
Now we assume that the developer made a bad decission and calculates the total amount on the client side and does not validate it any further on the server side, this amount is than passed to an online payment service.
Now let's say we edit this total amount to something different then the original amount, as an example 0.01€, and pay this amount via the payment service and receive the products. Later on the site owner notices the difference and tries to sue you, what would be the outcome most likely? (keeping the ethical aspect out)
Different country's have different laws so I share my thoughts with you from the german view (Other countrys welcome!):
As far as I know to have a valid contract both sides have to give a "willing declaration" (Willenserklärung) following the pattern
Offer + Acceptance = Contract
So is a dynamically calculated price shown on your screen a valid offer you accept by submitting the digital form?
Also one could argue that editing the field and sending it to the server makes it a completely new offer (stating what you're willing to pay) to the company and since they redirect you to the payment service without further notice they accept it.
I would say this falls under an implied acceptance (Stillschweigende Annahme):
Offer + Direct payment without further exchange = Receipt of Products (eg in a store)
To set some boundaries let's say that the amount is shown as Text and not in a editable input box and the amount send to the server in some way hidden on the client.
I don't know much about legals so feel free to ask if you need any other information