Why did they say "Ein Lanu Kesef v'Zahav Im Sha'ul"?
Rashi: David tried to appease them with money; they did not accept.
Malbim: Through Sha'ul's decree, Yisraelim took their money. From Yisrael they claimed only the money; from Beis Sha'ul, they claim only Nefsshos.
Why is it written Li, and we pronounce it Lanu?
Rashi citing Yerushalmi Sanhedrin 6:7: After they refused to be appeased with money, David thought, perhaps some are embarrassed in front of one another [to say that he wants gold and silver]. He went to each to appease him alone, and he did not accept.
Radak (5): Sometimes the verse speaks in the plural regarding every individual, and in the singular regarding the Klal (the entire nation).
Why did they say "Ein Lanu Ish Lehamis"?
Rashi: We do not want to kill anyone else in Yisrael; they did not sin against us.
Malbim: We have no capital claim against Yisraelim (only the money that they took from us).