Or HaChaim on Genesis 18:30/אור החיים על בראשית י״ח:ל׳

1 ויאמר אל נא יחר לה׳ He said "Let my Lord not become angry." The reason that Abraham was concerned about G'd becoming angry this time was because he tried to add his own merit to make up the quorum of ten in one of the towns in case this was all that was lacking, whereas G'd would make up the quorum in the other towns. Abraham used the word נא again because he introduced a new element, i.e. use of his own merit as a protective factor. [The author may have deduced the principle that Abraham injected his own merit here because of the absence of the word אליו when he speaks to G'd. I believe the novelty of the author's approach is that he believed that throughout the dialogue reported by the Torah Abraham did not abandon the attempt to save all five cities. Ed.]