Marriott does have a Best Rate Guarantee.

"If you find a lower rate for the same hotel, room type and dates within 24 hours of making your reservation, submit a Best Rate Guarantee claim form. We'll review the claim, and if approved, we'll match the rate and give you an additional 25% discount on the room or 5,000 Marriott Bonvoy™ Points…it's up to you."

Claim form:

https://www.marriott.com/look/claimForm.mi

Cheers.