Fetching details from the dictionary...

something thrown in (with a purchase); something extra; free gift; freebie; giveaway; premium; prize
(giving a) discount; price reduction
addition; embellishment (e.g. to a story)
そのうえお負けに風邪まで引いてしまった。
On top of that, I even caught a cold.
お店でお負けをしてもらったので嬉しかった。
I was happy because the store gave me a discount.
お負けをつけてくれると客は喜ぶものだ。
Customers are happy when you add a bonus.