Musar sobre Terumot 1:18

The commentary of Rashi above that the amount of Isaac's harvest is described in order to enable one to "correctly estimate" the tithes, appears to contradict the Halachah. We know that tithing must never be based on an estimate, but must be measured exactly (Maimonides Hilchot Maaser 1,14). Even if the amount tithed exceeds ten percent of the harvest, one did not fulfil the commandment if one has estimated instead of measured the required amount. We must therefore assume that Isaac measured the tithe, but added the תרומה גדולה by estimating the amount, and this is what the Midrash refers to. Similarly, the Mishna Terumot 1,7 says that one does not give תרומה by measure but by estimate from the total amount. The quantity of תרומה to be given as per our sages serves only as a guideline, not as absolute measurement. The scriptural allusion for this procedure is Numbers 18,27: ונחשב לכם תרומתכם, "your gift will be 'considered' for you." This means that the process depends on מחשבה, thought, rather than measure. Although the verse mentioned is written in connection with תרומת מעשר, we have a principle that if a statement is not needed where it appears we may apply it elsewhere, in this case we apply it to תרומה גדולה.