I'm surprised you hadn't heard of it. There is a discussion of it in Maury Klein's well documented book Days of Defiance. Five Carolina congressmen put together a letter in December 1860 prior to secession in response from a request by Buchanan that their proposal previously presented to him be in writing. They had proposed that the status quo be maintained with regard to the forts and that the matter be resolved through negotiation.
The South Carolinians verbally explained that any shift of troops by Anderson from Fort Moultrie to Fort Sumter would violate their proposal. Buchanan said that his policy was to maintain the status quo but that he couldn't pledge anything. He said that it was a "matter of honor among gentlemen."
From Klein: "All parties went away believing a bargain had been struck, without knowing exactly what the bargain was, who the parties to it were, and what its precise terms were."
Buchanan's reaction to Anderson occupying Sumter was interesting. From Klein again:
...The next morning when Trescot was readying their credentials [the credentials of the South Carolina commissioners who had come to talk with Buchanan], Louis Wigfall burst in with a telegram that Anderson had spiked Moultrie's guns and moved to Sumter. The commissioners and Trescot were stunned. "True or not," said Trescot amid an animated discussion, "I will pledge my life that if it has been done it has been without orders from Washington.
Just then Floyd arrived. He blanched at the news and confirmed what Trescot had said, that such a move "would be not only against orders but in the face of orders." ....
Trescot informed Senators Jefferson Davis and Robert Hunter and went with them to the White House to demand an explanation. Buchanan had not heard of Anderson's action.
Buchanan slumped into a chair. "My God!" he cried wearily. "Are calamities ... never to come singly! I call God to witness -- you gentlemen better than anybody else know that this is not only without but against my orders. It is against my policy." ...
Indeed. In his letter to Barnwell, Adams and Orr on December 31 he refused to condemn Anderson's actions, pointing out that the major was under orders to take whatever steps were necessary to ensure the safety of his men and he wasn't going to second-guess him. I'm not aware of any public statement ever made by Buchanan where he condemned the move.