Swim back to the 100 Year Old Wise Man and give back his amulet. Tomba will obtain the Old Man's Proof, which gives him 100,000 . The event is cleared.
The amulet is located near the end of the water channel. Once there, press the jump button while Tomba is in mid-air to make him dive into the water and collect the amulet. Note that the event can also be activated by collecting the amulet before talking to the 100 Year Old Wise Man.