Is there some way in which User A who could load a transaction onto the block chain and if some other User B who is willing to pay User A, without User A being online could release the loaded transaction. Something like a smart contract ?
It's not entirely clear what you are asking.
To start with, it is the miners (or mining pools) that load the transactions onto the blockchain. Users typically just broadcast the transactions to their connected peers, and then wait for a miner (or pool) to confirm the transaction by loading it onto the blockchain.
So, here's what I think you might be asking:
- User A is not online
- User B is online
- User A wants to send a bitcoin payment to User B in exchange for a local currency payment from User B
This can be accomplished in a variety of ways as long as User A has some method of giving data to User B.
For example if User A has his private keys, and has access to an offline computer, and already has the list of unspent outputs that he can control with his private keys...
Then User A can build and sign a transaction that pays a bitcoin address given to him by User B. He can then give that transaction to User B. User B can then validate the transaction and broadcast it onto the network to make sure it gets confirmed.
Or if User A already has an address that has received the exact amount that he wants to give to User B, then User A can just give the private key to User B. User B can then import the private key and immediately create and broadcast a transaction that sends those bitcoins to some other address that he controls.
There are other methods as well. It really depends on exactly what you're trying to accomplish.
