No, you can't negotiate what it costs to redeem the night from Marriott. But you can certainly negotiate the effective cost with the property agreeing to post points to your account, in association with the redemption. Pay 20K points to Marriott and have the property post 5K points to my account - that's effectively a 15K redemption IMHO, pedantics aside.

I'd worry that too many points added to my account with "customer service adjustment" or similar terminology could flag me as a chronic complainer.

I also suspect that agreements with th property to return points on award stays would violate the property's own contract with Bonvoy and thus be unenforceable if problems were to arise.It would also seem to violate the Bonvoy T&C in that point requirements for various types of redemptions are listed. Regardless, it doesn't seem ethical to try to do this.

Ancient history but the few times I tried it as a very long-term solo guest were not worth the effort. Limited price concession once or twice, and one or two managers got quite irate