Answer:
Trade-in allowance
Explanation:
A trade-in allowance is a type of discount in which the price of a good is reduced by the value of a another good that the buyer gives to the seller.
In this question, we have a trade-in-allowance because buyers give a product (a used vacuum cleaner) valued at $100 in exchange for a discount by the same amount of the total price of the new vacuum that they want to buy.