Theory:

     “A bird’s all right.” The customer pointed suddenly to a suspended cage which contained two snowy birds. “Doves? How much for those?”

     “Five-fifty,” came the prompt answer. “And a very reasonable price. They are a fine pair.”

     “Five-fifty?” The man was obviously crestfallen. He hesitantly produced a five dollar bill. “I’d like to have these birds. But this is all I’ve got. Just five dollars.”

     Mentally, Mr Purcell made a quick calculation, which told him that at a fifty cent reduction he could still reap a tidy profit. He smiled magnanimously.
Explanation:
 
When Mr Purcell exclaimed, “A bird!” the man said, “A bird’s all right.” Later the man suddenly pointed to a hanging cage, and it contained two snowy birds. The term “snowy birds” refers to the birds’ appearance, which is similar to that of snow or white. Then he looked at the doves and inquired the storekeeper about the price for those pair of doves.
 
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A man pointed towards a suspended cage
 
After listening to the customer’s question, he instantly said, “five-fifty.” The storekeeper later mentioned that he had given a reasonable price for the fine pair of doves. After hearing the cost of a couple of doves, the man was unhappy and disappointed. He hesitantly produced a five dollar bill. Finally, the man expressed his desire to own these birds. On the other hand, he said that he only had five dollars with him.
 
Later, Mr Purcell did a quick mental calculation and discovered that he could still earn a good profit even with a fifty-cent reduction. So he smiled generously, thinking of the quick profit he was going to make.
 
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Mr Purcell thought about the profit he was going to make
 
Meanings of the difficult words:
 
S.No
Words
Meanings
1
MagnanimouslyIn a way that is kind and generous, especially towards an enemy or someone you have defeated
2
Suspended Supported by attachment from above; hanging
3
Prompt Done without delay; immediate
4
CrestfallenSad and disappointed
5 
Reduction A thing that is made smaller or less in size or amount
6
ProfitMoney that is earned in trade or business after paying the costs of producing and selling goods and services
Reference:
National Council of Educational Research and Training (2007). Supplementary. I Want Something in a Cage - L E Greeve (36-42). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.