This is one of the many things I find funny about people who say Lucifer manipulates Sam. Lucifer’s whole thing is he rebelled through anger at all his siblings just blindly doing as they were told. Lucifer is the ultimate advocate of free-thinking. He thinks the demons who have been fooled into believing he’s their ‘father’ who cares about them are funny, he totally dismisses them after killing a bunch: “what? They’re just demons”. Lucifer thinks people who are easily manipulated are awful. All he damn well wants is for his siblings to learn to think for themselves. He claims he was cast down “because I had a mind of my own”. He shows a fondness for Castiel, even though Cas doesn’t agree with him, but at least Cas isn’t just another drone, he can think for himself!
And then there’s Sam, who is a complete parallel of Lucifer. He’s always been the one with the different opinions, not like Dean in just blindly following their father. Sam’s a fellow individual, another independent mind, the kind of person Lucifer appreciates for being like him in being able to question injustices. Lucifer just wants to help Sam to keep using this insight. He doesn’t need to manipulate Sam, because he truly believes that if Sam were to use this independent mind of his to look at the situation with logic and an unbiased mind- without Dean, without the fixed concept of right and wrong, without the predetermined label of him being ‘the devil’- Sam will come to the same conclusion as him and willingly side with him. That’s why all Lucifer does is talk to him and explain himself. Lucifer doesn’t manipulate Sam. Lucifer reasons with Sam. Lucifer doesn’t care for his demon army, because he considers them below him, but Sam is made for him, worthy of him, and as his other half, Lucifer treats him with the respect of someone as intelligent as himself. There’s a reason Lucifer says “any of this sound familiar?” because he considers them on mutual ground.
Lucifer isn’t certain from the start that Sam will say yes because he has confidence in his own manipulative abilities. He’s certain because he has confidence in Sam’s intelligence.