קבלה על כתובות 99:18

It appears to me, once we know that the previous tractate [ed. see introduction to Tractate Terumot above] corresponds to the 12 tribes, and also since it is well known that in all the Midrashim the Tribe of Levi is called "maaser" from the tribes (being as Jacob swore an oath to "give maaser from all that [God] gives me", he therefore separated Levi as "maaser"). Based on this, I feel forced to ask a question: In the Talmud, Tractate Ketubot (50a) teaches that a person should not give more than a fifth of his wealth to charity and brings a proof from Jacob who said "Aser Eh'asreinu" which the Talmud interprets to mean a fifth [ed. as opposed to merely a tenth as the simple reading implies]. If this is so, how could Jacob have fulfilled his oath by only sanctifying Levi [ed. which would be one tenth of his sons] when he vowed to sanctify one fifth [ed. ie. two children out of ten]?