The משנה says that if a person says he will be a נזיר but only על מנת that he can drink wine, he is considered a נזיר and he can’t drink wine. רבינא says that even רבי שמעון agrees to this since he is a מתנה על מה שכתוב בתורה. תוספות in ד"ה דהוי asks that נזירות is something that cannot be done על ידי שליחות. If so, we have a rule that says מילתא דליתא בשליחות ליתא בתנאי, so even if he wasn’t a מתנה על מה שכתוב בתורה the תנאי should not work since נזירות is ליתא בשליחות. He answers that since a נזיר can send his קרבנות through a שליח, it is considered איתא בשליחות. רבי עקיבא איגר asks here in the גליון הש"ס that we see on דף פּin נדרים that if you say you will not wash yourself forever if you wash today, the תנאי works even though it’s ליתא בשליחות. He leaves it צ"ע. The קרן אורה has some other very important הערות. First, why would the fact that you can send קרבנות with a שליח mean that נזירות is considered איתא בשליחות? (Interestingly, the שיטה מקובצת says what תוספות says and adds that "עיקר נזירות הם הקרבנות"). Moreover, it's not like he can even bring his own קרבנות for you without you being קונה them. All he can do is bring your קרבנות for you. He suggests his own two answers that תוספות could have answered to his question. First, this is in רבינא, and רבינא happens to hold that תנאי works even by something that is ליתא בשליחות, as we see by חליצה that he holds תנאי works even though it is something that is ליתא בשליחות. The קרן אורה doesn’t like this answer since we then bring a ברייתא that supports רבינא so everyone will have to agree that a תנאי should work if not that it was מתנה על מה שכתוב בתורה. His second answer is that you don’t need משפּטי התנאים by נזירות since by נזירות there is a rule of פּיו ולבו שוים so if it isn’t פּיו ולבו שוים then it isn’t a נדר even if there was no תנאי כפול or any other rules of תנאים. Another possible answer to תוספות’s question is brought in the שיטה מקובצת by ר' עזריאל ז"ל who says that we only require something to need שליחות if the תנאי is coming to uproot a מעשה. However, נדר ונזירות are not a מעשה so תנאי can work. Clearly, תוספות does not agree to any of these suppositions since that is not the answer he gives. To answer תוספות, the גרי"ז at the beginning of הלכות נזירותexplains that תוספות holds that while it's true that by נדרים you need פּיו ולבו שוים, that would just make it that if you don’t have פּיו ולבו שוים the נדר is not valid. However, what if you wanted the נדר to be valid, just with different rules? Then you need the גזרת הכתוב of משפּטי התנאים to have the תנאי modify the מעשה. So by every case of מתנה על מה שכתוב בתורהthe question is not whether you can annul the תנאי. Just the opposite--the question is whether we can keep the נדר alive and modify it via תנאי. To that we answer that in a case of מתנה על מה שכתוב בתורה you cannot. By רע"א’s case of a נדר to not wash ever if you wash today, there you are using the תנאי simply to annul the vow, and that would work without משפּטי התנאים so it wouldn’t matter that its ליתא בשליחות.