[robocat]

"Fully" is anything but fully. It is an unrealisable ideal.

Money is not fungible across currencies or across time or in different locations or a wide variety of different characteristics. Even 1USD at the same time in the same place can have have different values depending on how/where it is stored ($1 on credit card, $1 in USDT, $1 note at a place that doesn't accept cash, $1 silver coin, $1 in Argentina, $1 promised, etcetera).

Even "fuels of the same specs" are not really fungible. Only approximately on some markets. Russian oil? Fungible is an ideal and markets are designed to try and get closer to the ideal but markets and specs are nowhere near perfect.

[RandomLensman]

Two dollars in the same situation are fully fungible between each other - if they are in different systems, or even gold, silver, etc. they are not in the same situation or even money or dollars (but something of a dollar value). For example, dollars in a checking account are fully fungible, you would not know which specific dollar you get if you transfer one.

That situationally swapping between fungible things can be blocked etc. doesn't mean that they thing itself isn't fungible.