I wouldn't call it a bribe, I would call it stealing.

The hotel is supposed to collect additional revenue from a nicer room. The clerk pocketing a fraction of that revenue and the hotel getting nothing sounds a lot like embezzlement to me.

One time when I was a cashier at a little store, this guy offered me a small price for some item, suggesting that I not ring it up "wink, wink". I told him I was not going to do that. Wouldn't that be embarrassing if your bribe was rejected by an ethical desk clerk?