First of all, there is something Chazzal called an עבירה לשמה a violation with good intention, see Horios 10b and Nazir 23b. That act is juxtaposed with the opposite act, a fulfillment of a commandment with bad intentions. Hashem actually rewarded people for doing something wrong when their intentions were correct. (Please, any sabateans, don't misuse this concept). So even if your question is correct and somebody should not give up Olam Habaah to 'merely' save someone's life, he definitely shouldn't be surprised if he gets rewarded for it. I really don't think Yael (if we assume she was Jewish and relevant to this discussion) was expecting a great reward for sleeping with Sisra. She was probably assuming she would be punished (I.e. loose Olam Habaah) for that act. Ok, seven acts. And yet she was in fact rewarded with greater praise than the four mothers. (See later the Esther caveat, it applies here too).
In a similar vein when making the deal you mention, the person would only be trading his Olam Habaah he had earned up until that point. Ok. Clean slate. But now, he has the mitzvah of saving a life on his plate, which is like saving an entire world! I'd call that an upgrade for sure. (Unless this person has saved other lives beforehand. Than his Olam Habaah would probably stay the same. But as a free bonus, he saved someone's life!)
Next, since in your comments you have linked this to a type of physical suicide, i would point out that certain types of shortening of one's life for the sake of granting someone else life are not uncommon in Chazzal. Adam HaRishon gave away seventy years of his life to David HaMelech (Pirkei DiRabi Eliezer 19). I will suggest Esther's act as a source for this as well, at least as Targum understands her words in Esther 5 16 כאשראבדתי אבדתי, she was expecting to die saving the nation. (although one can argue a major difference was that there she was trying to save an entire community).
Also, the fact that you wrote 'merely' about saving someone's life is a bit telling that you don't give life the same exalted status as Hashem and Chazzal did. Perhaps delve into the subject and this question will no longer bother you. One starting point would be in the Igros Moshe who went wild on Reb Shlomo Kluger for suggesting one may avoid saving someone's life if it would embarrass the one doing the saving. (Remember, embarrassing someone is like killing them.) Sorry about the Mussar. Please don't downvote just cause of this. (shameless. I know)