Theory:

The sun glistened on the sand, and the sea waves broke waywardly.
A child sat playing with shells.
He raised his head and seemed to know me and said, “I hire you with nothing.”
From henceforward that bargain struck in child’s play made me a free man.
Explanation:
 
The speaker had by far rejected power, money and beauty, thereby showing that he was not interested in the materialistic things of the world. He wanted to be hired by someone who could offer something invaluable to him. He walked along, searching to strike such a bargain. In the previous stanzas, he searches for a job for almost a day. When he rejects the fair maiden, it is almost night. But he does not lose hope and continues his pursuit the next day. This shows that one must continue to seek what one wants, irrespective of what the situation is.
 
The next morning, the sun shines brightly, that its rays are reflected on the sand. The speaker seems to be walking near a beach, as the sun's light makes the sand glisten. The sea has waves that were difficult to be controlled, indicating that it was not a calm day. The state of unrest in the mind of the speaker is depicted here. As he walks along, he sees a child sitting by the beach. The child seems to be unperturbed by the sea or the surroundings. He sits there playing with the shells of the beach, enjoying his own company. He is very simple and does not have any possessions except for the shells. The child seemed to know the speaker already and it appeared as though he was waiting for his arrival.
 
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Beach shells
 
The child raises his head from what he had been doing and proposes to hire the speaker. Unlike the people who had already made this offer, the child says that he would hire the speaker with literally nothing. Generally, people look for more offers and benefits when they want to be hired. But the speaker immediately agrees and the bargain that was made in the last discourse struck as a fair deal to him, as it made him a free man. More than money, power or beauty, one's work or passion needs to make him free and not bound to materialistic possessions.
 
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The child makes a bargain in the beach
 
Meaning of difficult words:
 
S.No
Words
Meaning
1
GlistenShining
2
WaywardlyDifficult to control
3
HenceforwardFrom now on; In the future
4
UnperturbedUndisturbed
5
BargainTo make an agreement
6
MaterialisticFocusing more on material possessions; Money-oriented
Reference:
National Council of Educational Research and Training (2008). Honeydew. The Last Bargain- Rabindranath Tagore (pp.73-74). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.