No where in Srimad Ramayana it was mentioned that Vali had a boon that he will take 50% of power from his opponent, who ever challenges him. However, it was mentioned in Ramayana that he is very powerful and fearless.
According to Srimad Valmiki Ramayana, Vali had always been wearing a lustrous necklace, given by his father Lord Indra, on his neck. It was only when he is breathing his last that he asks Sugreeva to take it before he passes away.
Sri Hanuma shows immense strength and dexterity in SUNDARA KANDA. He even says that he alone can slay Ravana alongwith his army. Even Ravana acknowledges Sri Hanuma’s capabilities in Sundara Kanda (Sarga 46 Sloka 11 ).
दृष्टा हि हरयः शीघ्रा मया विपुल विक्रमाः |
वाली च सह सुग्रीवो जाम्बवान् च महाबलः ||
नीलः सेना पतिः चैव ये च अन्ये द्विविद आदयः |
न एव तेषाम् गतिः भीमा न तेजो न पराक्रमः ||
न मतिः न बल उत्साहो न रूप परिकल्पनम् |
"Earlier, I saw the Vanara-s of a great prowess like Vali and Sugreeva, the mighty Jambavan, Nila the Chief of army and so on like Dvivida.”
“Their scheme of performance is not so fearful as this; nor their efficacy, nor their prowess, nor their intellect, nor this capacity to change their form or energy at will."
A question arises as to why Sri Hanuma, having so many capabilities, remained as a silent spectator, when Vali banished Sugreeva and grabbed Ruma, the wife of Sugreeva. Could not he stop Vali with his might?
He could have, I think. From the various situations in which Sri Hanuma’s character was depicted, I think, Sage Valmiki projected him to be a KARMA YOGI and STHITA PRAGNA.
Being a wise person he knows that if he defeats Vali and hands over the Kingdom to Sugreeva, the latter may not like to take it as till then Sri Hanuma had been his minister only.
A kingly person will not accept such a gift from a minister, as it will be viewed below his dignity. That is why Sri Hanuma remained silent and keeps on following Sugreeva.
However, accepting help from another king/would be king is a different matter. It is common among kings.
Hence, Sri Hanuma gladly accepted the proposal of Lakshmana to arrange a meeting with Sugreeva, understanding that Sri Rama can slay Vali.