Which short story, please help:

I remember that it is about a baker's shop or some other kind of store. The owner - a helpfull woman - has a regular custommer, that always only ask for old, dry bread. She thinks he is poor, and one day she puts some buttter on the bread, without telling him. He comes back furios, because he was using the dry bread to erase lines from his drawings (I think is an architect or something the like), and now the butter has spoiled one of his drawing, that he used many months of work.

I would be so happy if you could give me the title.

Best regards