‘Then Joseph said, “Behold I have bought you this day, and your land, for Pharaoh. Look here is seed for you, and you shall sow the land, and it shall be that at the ingatherings you shall give one fifth to Pharaoh, and four parts shall be your own, for seed of the field, and for your food and for those of your households and for food for your little ones.” '
The famine is now approaching its end and Joseph declares their new position. From now on they are debtors to Pharaoh for one fifth of their produce, and on these terms he provides them with seed for sowing. This is not onerous. It may well be that they had already been paying this amount in taxes. And to receive seed at the end of a famine was luxury indeed. This has ever been the problem of a famine, that the seed has been consumed and little is left for sowing.
“This day.” This clearly is not intended to mean that the transaction from start to finish took place on that day. These transactions took place over fairly long periods. ‘This day' refers to the end position. He is really saying, ‘this day I declare to you that ---' and from this day they must fulfil the responsibility of the fifth.
We can compare with this how later Israel would have to give one tenth to Yahweh as well as many sacrifices and offerings. One fifth is a typically Egyptian proportion.