How to correctly update more than 1 data in database

I'm trying to make a simple function that will charge cashback for purchases on the site. The principle is as follows: I have a buy table, where all purchase operations are indicated, and in this table there are 3 required columns: user_id, bank and status. Bank - transaction amount, status - transaction status, user_id - ID at users table. If status is 2, then the purchase has been paid. I take all the data from the buy table, get it with status = 2, and then I need to enter to the users table, find the user ID there by user_id from the buy table, see the amount of his purchase where status is 2, and if everything is fine, I need to add 1 % cashback of the purchase amount.

I'm trying to do it all with this function:

public function cashbackUser()
{
    $find_user = DB::table('buy')->where('status', 2)->get();
    $users = DB::table('users')->get();
    foreach ($users as $addCashback) {
        $addCashback->cashback = $addCashback->cashback + $find_user->bank / 100 * 1;
        $addCashback->save();
    }
}

But i received error: Property [bank] does not exist on this collection instance.. How i understand, it can't find the all summs from bank at buy table, because i can't to use ->get()? What i need to change and get work it?