How to access mapping array and address of contract in truffle test

I have 3 contracts: sender, receiver and transferTest. I am getting error in transferTest due to an address array declared in the reciver contract:

receiver.sol:

pragma solidity ^0.5.8;
contract receiver{
   address public owner;
   mapping(address => uint) balance; 
   constructor () public{
      owner = msg.sender;
   }
function() external payable{
   balance[owner] += msg.value;}
}

transferTest:

pragma solidity ^0.5.8;
import "truffle/Assert.sol";
import "../contracts/sender.sol";
import "../contracts/receiver.sol";
contract TransferTest{
function testingTransfer() public{
   sender senderObj = new sender();
   receiver receiverObj = new receiver();
   senderObj.transferTo(msg.sender, 10);
   Assert.equal(receiver.balance[receiver.owner], 9, "Received amount is not correct");
}
}

I am getting following error message:

> testingTransfer.sol:12:20: TypeError: Member "balance" not found or not visible after argument-dependent lookup in type(contract receiver). Assert.equal(receiver.balance[receiver.owner], 9, "Received amount is not correct"); ^--------------^
> 
> Compilation failed. See above. Truffle v5.1.67 (core: 5.1.67) Node v10.23.3



Also I used msg.sender for the address of receiver contract as the first argument of transferTo function. Please guide me how to get rid of compilation error and replace msg.sender with the address of receiver contract.

Zulfi.