Can I assume that someone can still send any amount to a receiving address with a blank amount?
And, what will be the result of sending more or less to a receiving address which has set a specific amount?
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Sign up to join this communityPayment requests in Bitcoin are completely out of band. They have no binding effect for what happens on-chain. An address merely communicates what locking script the receiver would like the sender to pay into. While a payment request might specify an amount, a sender could send any other amount as well. Once an address has been communicated, receiving is completely passive: the receiver does not have to sign off, nor do they get a chance to veto a payment.