I believe you have verses mixed up on this point.
Luke 15 says Jesus spoke to the Pharisees and scribes in parables.
But Luke 16, which is where we find the story of the Rich Man talking to Abraham, starts without such a mention - - and he is no longer speaking to the Pharisees:
And he said also unto his disciples, There was a certain rich man, which had a steward; and the same was accused unto him that he had wasted his goods.
Later in Luke 16, he derides the Pharisees. And then the tone of his words changes again:
Luke 16:17 quotes Jesus: “Whosoever putteth away his wife, and marrieth another, committeth adultery: and whosoever marrieth her that is put away from her husband committeth adultery.”
And then he begins the story of the Rich Man.
Naturally, I must conclude that the text is a parable… but not because Luke says it was a parable!