Which of the following events corresponds with a sales return?

a$.)$ After suffering the loss of her fennec fox, lan agreed to buy back his customer's unused litter.

b$.)$ Ian had a sale on prairie dog food: Buy one, get one half off.

c$.)$ lan sent a specialty cage supplier a payment on account.

d$.)$ lan gave a customer a $20\%$ discount on a marmoset cage.