• GST on forfeiture amount for surrendering flat and Non refundable tax

Dear Sir,
I had booked a flat in Godrej Oasis. After the initial booking, they have reduced the cost of the flat substantially. In view of this, I had requested the Builder to surrender the flat. After a wait of almost ten months, they have agreed for the surrender, however, over and above the forfeiture charges, they are also deducting following taxes:- 
a. Non Refundable Tax. The reason given: Service Tax paid on canceled invoices needs to be claimed as Refund under the Sec 11B of Central Excise Act, vide Sec 83 of the Finance Act 1994.Section 11B(1) of Central Excise Act, 1944 states that any person claiming refund of tax may make an application for such refund before the expiry of one year from the relevant date. Further, as per the explanation given to the above section, the relevant date in our case shall be the date of payment of duty or tax. As per the above provision it is clear that GPL will not be eligible to claim a refund of Service Tax from the Central government as a time limit to claim service tax refund has lapsed.

b. GST on the Forfeiture amount. The reason given: As per section 34(2) of CGST Act, 2017, Any registered person who issues a credit note in relation to a supply of goods or services or both shall declare the details of such credit note in the return for the month during which such credit note has been issued but not later than September following the end of the financial year in which such supply was made, or the date of furnishing of the relevant annual return, whichever is earlier, and the tax liability shall be adjusted in such manner as may be prescribed.

Question: Are they entitled to deduct these tax from my principal amount? 
If yes, what is the rate of GST on the Forfeiture amount? 
Can they be claimed back? 

Thanks and regards. 
Col M K Choudhary (R)