Ass, gas, or grass.
Nobody rides for free...
An ideal situation in which to use barter, perhaps? Consider the following example.
Let's say I just mowed, so I have a surplus, ready-to-use supply of (c). Maybe I can get some (a) by trading some (b) that I produced from (c).
Sounds like a deal to me! :-D
(I can't believe I just typed this out loud).