UserToken.sol

contract UserTokenFactory {
  mapping(address=>UserToken) public userTokens;
  
  function getUserToken(address owner) returns(UserToken) {
    if(userTokens[owner] == 0) {
      UserToken token = new UserToken(owner);
      userTokens[owner] = token;
    }
    return userTokens[owner];
}

contract UserToken {
  UserTokenFactory factory;
  address public owner;
  mapping(uint256=>uint256) public balances;
  uint nonce;

  event TokenIssued(uint256 tokenId, uint256 quantity);

  constructor(address _owner) {
    owner = _owner;
    factory = msg.sender;
  }
  
  function issue(uint256 amount) returns(uint256) {
    uint256 tokenId = keccak256(abi.encodePacked(owner, nonce++));
    tokens[tokenId] = amount;
    return tokenId;
  }
  
  function transfer(address recipient, uint256 tokenId, uint256 amount) {
    require(owner == msg.sender);
    require(balances[tokenId] >= amount);
    balances[tokenId] -= amount;
    
    UserToken recipient = factory.getUserToken(recipient);
    recipient.recieve(tokenId, amount);
  }
  
  function receive(uint256 tokenId, uint256 amount) {
    UserToken sender = UserToken(msg.sender);
    require(factory.userTokens[sender.owner] == msg.sender);
    require(balances[tokenId] + amount >= amount);
    balances[tokenId] += amount;
  }
}