Sforno on Genesis 26:12/ספורנו על בראשית כ״ו:י״ב

1ויזרע יצחק בארץ ההיא, in accordance with G’d’s instructions “גור בארץ הזאת” (verse 3)

2מאה שערים. In accordance with G’d’s promise to Avraham in Genesis 12,2 ואהיה עמך ואברכך, “I will be with you and bless you,” i.e. with material blessings

3ויברכהו ה, with money. He sold his crops for cash. According to our sages as quoted by Rashi, the estimate of the size of Yitzchok’s crops given here is to teach us something about tithing properly. [according to Bereshit Rabbah 64,6 the question is why Yitzchok, who must have been aware that blessing does not rest on items which have been measured or weighed, bothered to measure his crop. The answer is that one must not tithe by estimating, but by measuring. If he wanted to tithe correctly, he had to first measure the size of his crop. Ed.] G’d directed the blessing to Yitzchok on account of his concern to tithe correctly, something we also find reflected in Maleachi 3,10 “bring the full tithe into the storehouse and let there be food in My House, and thus put Me to the test-said the Lord of Hosts. I will surely open the floodgates of the sky for you and pour down blessings on you.”