John Gill Commentary 1 Samuel 17:9

"If he be able to fight with me, and kill me, then will we be your servants; but if I prevail against him, and kill him, then shall ye be our servants, and serve us." — 1 Samuel 17:9 (ASV)

If he be able to fight with me, and to kill me, then will we be
your servants
For which it does not appear he had any commission or authority to say; nor did the Philistines think themselves obliged to abide by what he said, since, when he was slain, they did not yield themselves servants to the Israelites:

but if I prevail against him, and kill him, then shall you be our
servants, and serve us ;
to which terms also the Israelites did not consent; nor did David, who engaged with him, enter the fray on such conditions.