A bonus to a buyer's agent is payment for your silence. It's disrespectful to the buyer under all circumstances. You agreed to work for the buyer with a contract that stated how you would be compensated for ahead of time.
Think of it this way, does the bonus have value? Obvious answer is, yes. Then the bonus is only being used to sway your opinion. That's not in line with your code of ethics, where you're suppose to act in the buyer's best interests, not yours.
Some argue that the bonus is being paid by the seller, or the builder, or the developer. Hogwash. The buyer pays for everything in every real estate transaction. The assertion that they give money to a seller who then purportedly pays everyone else, and thus the seller is paying for your commission, bonus, title insurance, home warranty, etc., is false.
Now, what to do with the situation. That depends on how you think the seller is going to view you giving the bonus to your buyer. If you think it will help, then tell the seller that. If you think it won't, then agree with the buyer that you're going to rebate to them the bonus, put it in writing, and continue negotiating with the seller. The fact that the seller thinks you're going to keep the bonus can work to your advantage.
Best of luck,