Well, I have a sort of "non-answer". And that is that in common usage for those 2 exact examples you don't typically refer to an "offer" (or whatever you want to call it), you refer to the opening/position itself. Especially since no actual offer has been made.
Thank you, but I am not interested in your position.
I am considering two other openings at the moment.
A head-hunter contacting you about a position does not in any way imply that you are any closer to an offer than if you had contacted them about it, so no "tentative/prospective offer" language is needed.
I once worked for a company that had such a division (what I refer to as "head-hunters", staffing experts that sought out potential employees for a job opening). They contacted enough people to hold full rounds of interviews, you had no more likeliness of getting the job just because it was one that they were contacting you about vs. one you applied for. In fact, they purposefully would find very lowly qualified people and send them in to the interviewers to up the likelihood of the company taking someone (and therefore them getting their commission for finding them) by making the regular people look more qualified by comparison. So those people definitely were not any closer to a real offer, they were basically sacrifices.
If some sort of unofficial/spoken offer has actually been made (the OP does not specify it has, but for the sake of thoroughness), then it would technically would be called exactly that: an unofficial offer, and it would be completely appropriate to just refer to it as an "offer".
Thank you, but I am not interested in your offer.
Provisional offer does not fit here at all, because that is an actual offer that has been made but with provisions/conditions. That does not seem to be the case.
Prospective offer and potential offer fits the description the OP gives (i.e. an offer is expected but not yet actually given), but not at all the examples given. You wouldn't tell them you're rejecting their prospective offer that hasn't even been given yet, you'd tell them you reject the position (like the beginning of my answer). Likewise you wouldn't tell one potential employer you have "prospective offers" elsewhere. Until an offer has actually been made you'd still just say that they are other "openings" or "positions" you're interested in.
That's what makes this question a little hard to answer: the description and the examples don't actually fully mesh. You tend to be answering for one or the other.