In American History, what was "The Walking Purchase" ?

In the time of William Penn, there was a program called The Walking Purchase. An agreement with the Native Americans, at the time referred to as "Indians", whereby a white man could purchase land from the indians by walking an area in a day. Beginning in the morning, if he could walk the perimiter of an area and return to his starting position by nightfall, the area was given to him. My question is, what did that man have to give to the indian tribe to purchase that area of land?