Builder false promises causing me worries

I bought 2 flats(1BHK) on a single floor(first).

Afterwards, the Builder approaches me and tells me that he wants to 1 of the flat.

He says that he will give me another one on the tenth floor.

I said OK.

Builder then transfers money in my bank and I sign over one of my flat to him.

I then buy the flat on the 10th floor. All the money transfers back to builders account.

Builder promises that I will not need to pay any tax on the money I received as I have just changed the flat.

But a friend of mine says that I will need to pay tax on it.

Please help clear this confusion.

I don't have money to pay the double tax.