That you can exchange something for other currencies doesn't make it a currency. Currency is defined by its use as a medium of exchange.

Non-redeemable purchasable tokens could meet the definition of a currency. Conversion can be facilitated by exchange between users, rather than by the issuer. In this case, the issuer's sale price only sets an upper bound on the exchange rate.

> It's the one that makes Libra a currency.

it doesn't. Currency needs to have one other property - the legal requirement that a party accepts it as payment. For example, i cannot force a shop to accept my gold coins as payment (they could choose to accept it - but it's not a legal requirement). However, if i offered cash, they are legally required to accept it as payment.

Facebook can make this a pseudo requirement by virtue of network effect, but it isn't the same as currency with legal tender status.

US federal reserve notes don’t meet your definition - they can be legally refused for purchases, it’s only debts where you can’t refuse them. In a purchase scenario, where there is no pre-existing debt, a dollar is no more legally privileged as a payment method than pocket lint.