IERC20 Approve is not working

I'm facing error while using approve() in my contract, It is working fine with the functions provided by the ERC20 contract. But when I try to write my own function to do so, it is not working.

Here is my code

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

import "@openzeppelin/contracts/token/ERC20/IERC20.sol";

contract Testx {

    event Approval(address owner, address spender, uint256 value);
    event Transfer(address from, address to, uint256 value);

    uint256 amount = 100000;

    address public token;
    address public buyer = 0x5B38Da6a701c568545dCfcB03FcB875f56beddC4;
    address public seller = 0xAb8483F64d9C6d1EcF9b849Ae677dD3315835cb2;

    constructor (address _token) {
        token = _token;

    function approveAmount() public {
        require(msg.sender == buyer, "only buyer can approve the transaction");
        IERC20(token).approve(address(this), amount); // not working
        emit Approval(buyer, address(this), amount);

    function transferFund() public payable {
        // condition: when the fund should be transfered from buyer wallet to seller wallet
        IERC20(token).transferFrom(buyer, seller, amount); // working
        emit Transfer(buyer, seller, amount);

can anyone know why IERC20(token).approve(address(this), amount); is not working as intended ?

Thanks in advance....

I've invested my lots of time debugging it, but not be able to solve it.