BankAccount.cs

public class BankAccount
{
    public Guid OwnerUserId { get; private set; }
    public decimal Balance { get; private set; }

    public BankAccount(Guid ownerUserId, decimal initialBalance)
    {
        OwnerUserId = ownerUserId;
        Balance = initialBalance;
    }

    public void Withdraw(decimal amount)
    {
        if (amount > Balance)
        {
            throw new InvalidOperationException("Insufficient funds.");
        }

        Balance -= amount;
    }
}

public class BankAccountService
{
    public void WithdrawFromAccount(BankAccount account, decimal amount, Guid currentUserId)
    {
        if (currentUserId != account.OwnerUserId)
        {
            throw new UnauthorizedAccessException("You are not allowed to withdraw from this account.");
        }

        account.Withdraw(amount);
    }
}