solana-program-examples/basics/transfer-sol/native/program/src/instruction.rs

use solana_program::{
    account_info::{next_account_info, AccountInfo},
    entrypoint::ProgramResult,
    program::invoke,
    pubkey::Pubkey,
    system_instruction,
};

pub fn transfer_sol_with_cpi(accounts: &[AccountInfo], amount: u64) -> ProgramResult {
    let accounts_iter = &mut accounts.iter();
    let payer = next_account_info(accounts_iter)?;
    let recipient = next_account_info(accounts_iter)?;
    let system_program = next_account_info(accounts_iter)?;

    invoke(
        &system_instruction::transfer(payer.key, recipient.key, amount),
        &[payer.clone(), recipient.clone(), system_program.clone()],
    )?;

    Ok(())
}

pub fn transfer_sol_with_program(
    _program_id: &Pubkey,
    accounts: &[AccountInfo],
    amount: u64,
) -> ProgramResult {
    let accounts_iter = &mut accounts.iter();
    let payer = next_account_info(accounts_iter)?;
    let recipient = next_account_info(accounts_iter)?;

    **payer.try_borrow_mut_lamports()? -= amount;
    **recipient.try_borrow_mut_lamports()? += amount;

    Ok(())
}