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 ?

Lets say if A and B wants to exchange some bitcoin transactions between each other. But they both dont know each other before. A list his address some where and when B sees it, B will issue a bitcoin transaction to A.

A might not be online when B has issued this transaction. But B should receive back a transaction from A, if B has payed to A's address. How's it possible ?