From PhalkeFactory

Satyabhama was vain in her beauty, and apparently, in her sense of her importance in Krishna's life. The mischievous Narada decided to entertain himself. He convinced her to take on a ritual where she gives Krishna away to Narada and gets him back, when she gives Narada his weight in gold and other precious material. This ritual was called Tulabharam.

Why did Satyabhama agree?

Krishna was quite delighted to be the object of trading.. he sat on the weighing scales. Of course, all the gold, all the riches that Satyabhama owned, would not suffice to weigh against Krishna. The other wives also complied and gave all their gold and riches. To no avail. Finally Satyabhama swallowed her pride and went to Rukmini, who she was, in her heart of hearts, jealous of..Rukmini came with one leaf of the tulsi. The tulsi leaf, sign of her simple heartfelt devotion to Krishna, tipped the scales. Krishna watched kindly as Satybhama took this in, and learnt from it, the difficult lesson of humility.