If I go to a store to buy food and my friend says to me “buy me a bag of chips.” I agree, and buy him the chips. However, on the way home I lose the chips. Does he still have to pay me for them?
He has to pay for buying the chips (because of arguments such as arvus and motzi hotza’os al piv), but you have to pay him for losing them (it sounds like a case of peshiya).
The value of the chips in your car is not much less than chips in the store, and therefore the two obligations cancel each other out, and he does not have to pay you.