Musar for Berakhot 70:30

If you look closely you will find that what Rashi writes about "estimating," applies to all kinds of tithes, and not only to תרומה גדולה. Since Rashi writes that the estimate was based on how much the field was expected to yield, it appears that the method of estimating applied to all kinds of tithes, and not just to the מעשר ראשון. We may therefore conclude that already prior to the harvest Isaac "estimated" how much he would have to set aside as a tithe. Once the harvest was in the barn- at which time the obligation to give a tithe became operative, he measured carefully how much one tenth would amount to. Anything one eats from a crop before it is brought into the house or barn is not subject to tithes. Isaac, as we have shown, was especially careful to measure the grain even before anyone could have eaten from it. Since the verse under discussion deals with the commandment of tithing, it is appropriate for the Torah to add the words: "G–d blessed Him (Isaac)." We have a parallel verse in Malachi (3,10) when the prophet assures us הביאו את כל המעשר אל האוצר … והריקותי לכם ברכה עד בלי די, "Bring to Me all the tithes to the storage house,….and I will pour out for you endless blessings."

The linking of G–d and Israel in that Midrash is due to the fact that such theft is known in the Celestial Spheres. First man (whose father and mother was literally G–d Himself), had committed this crime of theft. The close connection in the Heavenly Regions between כנסת ישראל, and G–d is well known. It is there that Adam disturbed the equilibrium with his sin. This is why the kidnapper, גונב נפש, is mentioned in the Torah in connection with cursing or hitting mother or father.