Why don't you understand?
Has the original buyer has got registry done?
If not, can he get it done without paying VAT to the builder?
If the original buyer has not got registry done in his favour so far, how can the original buyer sell the property to you?
Therefore, here, instead of the first buyer getting the registry and then giving registry to you, YOU will be getting the registry from the builder straight. Therefore, the builder is asking the VAT amount from you. Otherwise, he will keep quite till such time the original buyer comes and demands from the builder for registry. At that time, the builder will ask for VAT payment from the original buyer.
If you speak legally, you have no right to ask anything from the builder. Legally speaking, you have not entered into any contract with the builder. Legally speaking, the contract is between the builder and the first buyer. Legally speaking, you are a complete stranger to the builder.
Therefore, if at all, you can ask only the original buyer, with whom you have entered into a contract to buy the ready built flat.
If the builder has to ask VAT with the original buyer, then the builder will ask the original buyer at his own sweet will and time. You cannot urge the builder.