The API should not support negative amounts/signed numbers. Amounts are typically reflected as positive, unsigned values, and their behaviour (i.e. debit/credit) should be governed by the transaction type itself.
Payment requests, along with fees, taxes, and discounts are typically modelled as separate transaction types (and/or as sub-types of a parent payment transaction). Refunds, and reversals (such as fee refund, tax refund, and discount refund) of those items should be different transaction types altogether, with a "credit" behaviour. However, these are likely outside the scope of the PaymentRequest, and are handled directly from the merchant to the customer.
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