Sometimes places run out of pesos, that's happened to us (though we are in PBA, I'd expect places in CABA to have a better peso supply).
Our last trip to CABA netted us a sealed block of 5 packets with 100 new 1000 peso notes each, with BCRA stamps, direct from the central bank, from a place on Av. Cabildo. They also do transfers to a bank account, which is very convenient and avoids having to queue in a bank for ages to deposit money.