A very strictly defined, easily dispersed IOU. None of this nebulous you owe me markers that range from my life to indentured servitude.

Hours of work will be traded for items. We can discuss hot many units of time can be traded for what items, and so forth. [Megamind refuses to be deeply in debt, especially so quickly.] Work may not guarantee results. I'm a genius, not a wizard, and materials are limited. Just that you will have my vast intellect for those hours I owe.

Are we clear?