Booking.com are running a promo where you can get $40 off a $80 hotel spend.
It seems that the $40 is credited back to your payment card after you complete your stay.
Not sure if the link contains any referrals or not, copied it from the deal posted somewhere else. Basically, when you click "Go to deal" and are logged in to your booking.com account there's a promo message at the bottom saying:
"Book now and get US$40 back!
All you have to do is book and you'll get US$40 back after your stay! Minimum booking cost US$80."
Terms and conditions: link
T & C are here https://booki.ng/2rT0zl2