Borges(?) Story - Would-be Assassin in Buenos Aires

In this story, a young man lives in a squalid room opening onto a patio in a poor section of town, perhaps near a large cathedral, waiting for a chance to assassinate an official as he comes out from Mass. He passes the time idling over a chessboard; two missing pawns have been replaced with a small coin and a bullet.

I think it takes place in the early 20th century; could have sworn it was by Borges, but cannot find it again.

Thanks for any suggestions.

Other than the Buenos Aires part (it takes place in Uruguay), I think you're looking for Borges' story, "Avelino Arredondo."

Thank you, Eva. Had not seen this story for 35 years.