Of course the money exists. Nearly all of it went to pay back Greece's original private creditors. Ask those institutions if the ECB money really exists, and they'll show you their balance sheets and loan books to prove you wrong.
Departing from you admitting the "proof" is a balance sheet, I'm not even saying that the money doesn't exist like every bank deposit -if people flock to all the banks to get their money out they'd get the bank system collapsing but not their money-; I'm saying that the money doesn't exist because it's just monetary creation
de novo. No expenditures were incurred to create that money but bureaucratic labour, and given the current state of astringency in the Eurozone it's more likely that money is going eventually to grease the gears of their economy and not to snatch its face value out of the economy via inflation.
A different situation with IMF's or EU's reserves. For instance, Argentina has 2,117 million SDR in the IMF, that is some 2,650 million euros. Those funds exists because, roughly, a third of it was contributed by Argentine taxpayers as direct funding, 5% or more was contributed by Argentine taxpayers as interests paid by Argentina on loans granted by the IMF, and the rest was contributed by other countries taxpayers through paying interests. So, some 1,000 million euros of that exist because Argentina exported wheat, meat, car parts and seamless steel piping in that amount and all the Argentines managed to split the bill among us via taxes.
That money also exists as an accounting entry, but it is far more real than these precise ECB's vouchers we're talking about.
By the way, Argentina doesn't owe any money to the IMF. They were thrown out years ago using a pile of money -mostly cheaply printed in the USA- gotten by exporting cars, soybean, glass and electric materials, so, the real deal.
And by the way, if Argentines were asked about what to do with our share of the IMF, we'd massively vote for "give it to Greece with interest rate 0 and tell them to return it the moment they can, there's no rush". And if the major 10,000 Argentine taxpayers were asked, I'm certain the outcome would be also a massive vote in the same direction.