Amazing thing about bitcoin that we take for granted is that it allows Alice to send sats to Bob while Bob is offline.
We need L2/L3 protocols on top of bitcoin to avoid high fee rates.
Cashu ecash is what I would consider an L3 protocols which allows Alice to send ecash with Bob, which a trusted mint acts as a bank.
But this requires Bob to be online! Tradeoffs!!
NOTE: Whoever the first to redeem the ecash token from the mint gets paid! So it has to be kept as secret between Alice and Bob, just like paper cash.
In internet we would like things to work asynchronously. That is even when Bob is offline, he must be able to receive ecash from Alice.
Cashu NUT-11 addresses this problem by facilitating the creation of ecash tokens which are locked to a public key, very similar to how bitcoin UTXOs are locked in Transaction output.
NIP-61 defines the format in which these payments can be stored in a nostr-relay. Bob has to publish his cashu pubkey, mints he trust etc., as a nutzap info event and Alice can publish the ecash tokens which are locked into Bob's public key as nutzapp payment event.
I have written a code example in rust using nostr_sdk and cdk 
GitHub
GitHub - delcin-raj/nip61-example-rs
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