truffle-test – 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 TestTransfer{

   function testTransfer() 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 replace msg.sender with the address of receiver contract.
==Update:Dec11 2021
My question is similar to:
Accessing Solidity Mapping

Zulfi.