[Ethereum] Transfer ERC20 tokens using another contract. tx.origin vs msg.sender

You cannot send tokens directly from one address to another address. Tokens balances only exist in the Token contract storage. You necessarily have to send a transaction to the contract indicating you want to make a tranfer, so the contract can update the balances accordingly.

I assume you are using Web3j and a deployed smart contract implementing ERC20 interface, where transfer method looks like this:

function transfer(address _to, uint256 _value) public returns (bool success) {
    require(balances[msg.sender] >= _value && _value > 0);
    balances[msg.sender] -= _value;
    balances[_to] += _value;
    return true;
}

You can use web3j wrapper for your contract to call contract methods on a blockahain. Suppose your Contract filname is MyContract.sol, then you'll need solc compiler and web3j tool and do the following in your console:

$ solc {contract}.sol --bin --abi --optimize -o {output-dir}/

this will generate .abi and .bin files from which you generate a java wrapper class:

$ web3j solidity generate /path/to/<smart-contract>.bin /path/to/<smart-contract>.abi -o /path/to/src/main/java -p com.your.organisation.name

This will produce a java wrapper class for your contract like MyContract.java on which you can call all the methods available on the smart contract:

import static org.web3j.tx.Contract.GAS_LIMIT;
import static org.web3j.tx.ManagedTransaction.GAS_PRICE;

//web3j initialization code goes here

//Load the deployed contract:
MyContract contract = MyContract.load(contractAddress, web3j, credentials,     GAS_PRICE, GAS_LIMIT);

//Call methods on the contract:
Future<> result = contract.transfer(_to, _value);

You can find more info on how to work with smart contracts wrappers here. Hope that helps.

edit. abi and bin for this particular contract can be obtained from etherscan.io You can transfer tokens even if you are not the owner of the contract. But you are only abe to transfer tokens available to your own account as seen in the contract:

 function transfer(address _to, uint256 _amount) returns (bool success) {
     if (balances[msg.sender] >= _amount 
        && _amount > 0
         && balances[_to] + _amount > balances[_to]) {
         balances[msg.sender] -= _amount;
         balances[_to] += _amount;
         Transfer(msg.sender, _to, _amount);
        return true;
     } else {
         return false;
     }
 }