Which is why the amount should be displayed on a display embedded in the card itself. The control for authorizing the transaction should also be part of the card.

Now, if only we carried around a device that included a display and some sort of input mechanism, plus a near-distance communication chip...

(Ok, if the device is a general computing device, a special secure operation mode might be needed for this sort of use case, one which can't be subverted by normally installed software, but still...)