One of the accused, identified as Harshal Ambre (26), is a former employee of the builder and allegedly roped in his friend Sumit Palanpula (20) to execute the plan, police said.
The duo, both residents of suburban Santacruz, were arrested yesterday from Vile Parle area when they were accepting Rs 25 lakh from the complainant.
A police official said Ambre used to work in the builder's office and was well aware about his business dealings.
According to the official, the caller who identified himself as "Chote" used to threaten the builder with dire consequences if he failed to cough up Rs 50 lakh.
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The investigation was taken up by the anti-extortion cell after the builder approached crime branch.
The official said that Ambre, who had worked with the builder for two years till 2014, was in financial crunch and hatched the plan to earn quick buck.
Ambre, working with a courier company in suburban Bandra, also got Palanpula involved in the conspiracy, the official added.