1. It is forbidden to inflict physical pain or damage to oneself or to one's friend [Rambam Hilchos Chovel Umazik 5: 1].

{Parenthetically, inflicting pain or physical damage to one's body is permissible in order to spare oneself from a more severe physical (e.g. resetting a broken limb) or severe emotional pain (e.g. elective plastic surgery).

Endangering one's life though, is forbidden, even to remove physical or emotional pain (e.g. dieting leading to anorexia) [Igros Moshe Choshen Mishpat II: 65, Riv'vos Ephraim VIII: 389].}

2. We assume that an average person in a balanced state of mind is unwilling to undergo extreme pain or loss of limbs unless it is the best of two worst options. As such, even when a victim says that he/she gives "permission" to an assailant to harm him/her, we assume he/she was not sincere, and the assailant remains obliged to pay for damages [Maseches Bava Kama 93a].

3. B was aware that A deposited a parcel of his food into B's property. B did not watch his/her animal from eating the food. B's animal ate the food and got sick. B cannot demand payment from A [Choshen Mishpat 393: 2, Sm"a 4].  

Application

 While Dave spiked Barry's food, Barry knew the risk involved and should not have eaten the food. 

Even if Dave told Barry to give him a blow, we assume he never meant to absolve Barry from paying for such an excruciating pain and significant damage.