Why are there no Brachot on Mitzvot which are "Bein Adam La’chaveiro"?
The Rashba (Rav Shlomo Ben Aderet, Spain, 1235- 1310) in his Responsa (1: 18) brings several rules for why we don't say a Bracha for these Mitzvot; 1. A Mitzvah that is not fulfilled through an act will not have a Bracha – for instance Shmitat Ksafim - annulling the debts on the seventh year 2. A Mitzvah that is dependant on the will of another and therefore may theoretically not be fulfilled after the Bracha is recited – will not have a Bracha – for instance Tzedaka. 3. A Mitzvah that is a rectification of an Avera, for instance returning a stolen item 4. Handing over that which belongs to him by the word of Hashem, for instance tithing (for more details see Har Tzvi Brachot 2)