CAPITAL GAIN

Since title was not transfered to Mr.A he will not be included in this picture at all. So it will be treated as direct sale by you to Mr.B.

Now the consequences of this:

1. Consideration and Stamp Duty:

You have received only 40 lakhs from Mr.A whereas now the sale value to Mr.B is 50 lakhs. You can treat the early receip from Mr.A as Loan, So if you make agreement for 50 lakhs you can return the 40 lakhs as loan repayment, and the other 10 lakhs to Mr.A(Mr.A has to offer it as income, probably as commission). In this case you have to pay additional stamp duty for short registration of 4 lakhs (54-50)

(or)

You can make the agreement for 40 lakhs and receive & settle to Mr.A and ask Mr.B to directly give the balance Rs.10 lakhs to Mr.A (Mr.A has to offer it as income, probably as commission). But the major issue here is you have to pay additional stamp duty for short registration of 14 lakhs (54-40).

2. Capital Gain:

Since the SDV of the property is 54 lakhs you have to consider the amount to be 54 lakhs even if you make agreement for 40/50 lakhs and pay capital gain tax on it after deducting the purchase and other costs if any (indexed cost in case of LTCG).

And Mr.B has to offer the short amount paid than the SDV under income from other source i.e., 14 lakhs (54-40) if agreement is made for 40 lakhs (or) 4 lakhs if agreement is made for 50 lakhs.