After a prophet's principled refusal to accept payment for a miraculous healing, his servant decides to exploit the missed financial opportunity. Acting behind his master's back, Gehazi speaks in his heart and plots his next move [רד״ק, מצודת דוד ושטיינזלץ]. He observes that Elisha deliberately prevented the healed man from giving his offering [מצודת ציון, מצודת דוד ושטיינזלץ]. Driven by a desire to amass wealth, acquire property, and elevate his own social status [אברבנאל], Gehazi begins to rationalize his impending betrayal.
To quiet his conscience, Gehazi relies on technical and legal justifications. He argues that there is no strict obligation to provide free medical healing to a foreigner, specifically emphasizing the man's Aramean origins [מלבי״ם]. In Gehazi's mind, while his master was correct to refuse a lavish gift so the foreigner would not associate the miracle with idol worship, collecting a basic medical fee remains perfectly acceptable. He even finds a loophole in his master's words. Because Elisha swore not to take anything directly from the man's hand, Gehazi convinces himself that acquiring something from his general possession would not technically violate the prophet's decree [אלשיך].
Committing to his plan, Gehazi swears by the name of God to chase after the foreigner and take something from him [מצודת דוד ושטיינזלץ]. This act stands in direct and tragic opposition to his master's behavior. While Elisha sanctified God's name in the eyes of the foreigner by rejecting material wealth, Gehazi's greedy pursuit and false oath profane God's name, effectively destroying the spiritual impact his master had just achieved [מנחת שי ואלשיך].
The consequence for this desecration is profound and exact. The word describing the "something" Gehazi swore to take is written missing a letter, allowing it to be read instead as a "defect." God punishes Gehazi measure for measure: because he swore falsely in his eager pursuit to take something of value, he ends up taking the man's physical defect. The leprosy of the man he pursued is transferred to Gehazi, clinging to him permanently [רש י ומנחת שי].