How can I forbid a client to change the currency and amount on the UI?

Q: How can I forbid a client to change the currency and amount on the UI?

A: Assign 'true' to the respective "lock" parameters.

For the Cashier widget:

data-currency-lock="true"
data-amount-lock="true"

For creating a server session:

"currency_lock": true,
"amount_lock": true,