You have two people in the center of an endless plane.
Each one is holding some item that can fit in their palm, like a tennis ball. Both persons are identical in physical ability and brainpower.
However, each one is just as clever as sly as the other one, and both are very clever and sly in their own right. They will immediately do what is best for them, and will always do what is best for them, unless there is an incentive.
Q: What foolproof way is there for these two to exchange their items?
Examples of inappropriate ways:
Throw it over each other: One might not throw it and chase the others item
Each hold the others item at the same time: One might let go first and the other won't let go, taking both items