> Perhaps we should just remove it, but then clients which don't know the URI scheme won't be able to show the amount to the user, which sucks.

If a client can't display the amount because it doesn't understand the scheme, it's better to tell the user that explicitly ("Unknown payment scheme.") rather than some vague warning that the value might not be accurate, while still displaying the amount as if it's correct. And where the client does understand the scheme, that value is redundant and should be ignored by the client anyway.

> Also, if we removed "display-amount", there's still a "label" field which could lie about the actual payment amount.

If we're going to have one potentially misleading value, let's go the whole way and have multiple? No.

It's possible the label could mention an amount, but it won't be displayed in the same way as the explicitly unknown transaction amount, and the sender would have to expect the recipient doesn't have support for the scheme, otherwise the mismatch will be clear.