Torah T'mimah - Bechukosai - selections

PROPERTY OF A WIFE THAT DIED
ויקרא פרק כו כ וְתַם לָרִיק כֹּחֲכֶם
מדרש רבה
זה המשיא את בתו ונותן לה ממון הרבה ולא הספיקו שבעת ימי במשתה לצאת עד שמתה בתו, נמצא קובר את בתו ומאבד את ממונו

In general, a husband becomes the owner of the wedding gifts after the wedding. Thus, in event of a death of the daughter immediately after the wedding, the father stands to lose his daughter and the money of the gifts for the wedding (נדוניא). Tosefos does honor the written request of the father that writes that the gifts to his daughter do not transfer to the son-in-law, if the daughter was to die soon after the wedding. The intent of the father is that the gifts to his daughter are for her benefit and not someone else.
Most recognize the concept that the husband inherits the wife is a rabbinical decree. Therefore, based on this Medrosh, Rabbenu Tam and the Rabbis of France issue a decree that if a wife was to die during the first year after the wedding, then the all the bride’s personal property would revert to her father or her legal heirs (not to the spouse). In the lands of Poland and the East, they decreed that if the wife was to die during the first three years of marriage, all her personal property would go to her heirs (and not the spouse). And if this occurred in the fourth and five years after the wedding, then the husband and her heirs would split the property 50-50.

REDEMPTION OF THE ANIMAL AND ADDING A FIFTH
ויקרא פרק כז יג וְאִם-גָּאֹל יִגְאָלֶנָּה וְיָסַף חֲמִישִׁתוֹ עַל-עֶרְכֶּךָ:
תורת כהנים זה בבעלים אמור
If the owner redeems the animal from Hekdesh, then needs to add a fifth (which means a fifth of the total sum after the addition value is added-in plain words one adds 25%, making the added value 1/5 of the total result (125%/25%)). This applies only to the owner, since the verse uses the word גאל, which is applicable mainly to the former owner, since anyone else would be acquiring (and would have used a word such as קנה).
It is proper for the former owner to add a fifth to the value since the object has more value to the former owner (sentimental or otherwise). Further, the mitzvah of redemption only applies to the former owner. Please note that Rambam at the end of Hilcos Terumah provides another reason.