Everything Kyle told Sarah has be to judged through the lens of who he was (i.e. a soldier, not a computer programmer) and what he was realistically likely to be aware of.
Humans didn't design T-800s, so he wasn't likely to be aware of all their tech specs or the full nature of their programming. He'd have been going from personal experience, or what he was told by John Connor or other Resistance fighters.
John would've known alot about Terminators, through his own personal experience with them as a child, from what his mother told him on the tapes she recorded for him, and from what she told him in person as well. However, he apparently withheld alot of this knowledge from Kyle, in order to avoid changing the course of events that led to his own birth.
For example, he didn't tell Kyle that he was going to reprogram a T-800 and use Skynet's Time Displacement Equipment (TDE) to send it to 1995, shortly after Kyle was sent to 1984. Kyle thought the TDE was blown up right after he went through, and John couldn't inform him otherwise without changing history and jeopardising his own birth.
In regard to Kyle's personal experience with T-800s, that was likely pretty limited. He told Sarah that they were a new model, and we only ever saw him fighting them, or fleeing from them. As far as we know, he never had a conversation with one, or watched one converse with other humans. Nor did he have the opportunity to see how one could grow beyond its original programming, given sufficient time to learn from its interactions with humans.
Those of us who've watched all the films almost certainly know more about T-800s than Kyle did. As such, we should take what he said about them with a pinch of salt, especially if it conflicts with what we've seen of them first-hand.
In Terminator: Dark Fate, we were introduced to 'Carl,' a T-800 running his own drapery business, who presumably engaged in transactions with customers and suppliers on a regular basis. That's pretty cut-and-dried evidence of the ability to agree to and abide by deals, and like the T-800 in the first film, Carl was sent back in time by Skynet (not humans) and was never reprogrammed by humans.
The only meaningful difference between Carl and the T-800 in the first film is the greater amount of time Carl spent learning from interactions with humans. And even the T-800 in the first film voluntarily paid for the hotel room he was shown using -- according to the official novelisation -- meaning that he engaged in at least one financial transaction that we know of, because it served his objectives to do so.
It was a four-storey firetrap that smelled of disinfectant and stopped-up toilets. In the winter it was a refrigerator, and in the summer, an oven, chilling or baking the human contents mercilessly. But it was cheap. Back from the main street. With a fire escape he could climb out of and into an alley, unseen by the desk clerk.
Therefore, he selected it, threw a wad of bills down onto the counter, and refused to sign the register. The steel-blue eyes fixed the big-eared, tiny-framed fifty-year-old clerk like a bug on a board. The clerk muttered something about writing Mr. Smith for him, then handed him the keys to the small room up the stairs.
This was his base of operations. It had to be secure; therefore, he could not bring attention to it by any overtly aggressive behavior, such as outright life denial. He knew enough about this society to avoid doing anything that would jeopardize this neutral zone. That was why he paid the desk clerk for the room.
The Terminator (novel by Randall Frakes)
On that basis, we can confidently say that not everything Kyle told Sarah about it being impossible to bargain with or reason with a T-800 was strictly true. That said, I suspect that when Kyle said that, he was probably thinking about it mainly from the POV of someone a T-800 had been programmed to kill.
So if a T-800 has been specifically programmed to kill you, then no, you probably can't convince it not to fulfill its mission via bargaining or reasoning. At least, not if it hasn't had sufficient time to grow well beyond it's initial programming.
But sure, if it has had time to grow beyond it's programming, then perhaps you could convince it not to kill you after all.
And if you're not the person it's been programmed to kill, or an apparent obstacle to its objectives, then sure, you could potentially strike a deal with it, provided it viewed that arrangement as one that was beneficial to its objectives.