Hi
1) It appears from your query that the customer has made only an advance payment which is technically an earnest money deposit given as a guarantee for fulfillment of a contract.
2) It also appears that the customer has failed to pay stage wise payment as stipulated in agreement of sale in the sense that he had failed to pay the monies for foundation level, slab level, brick level etc.
3) Technically an advance received from the prospective buyer, is an earnest money which is part of the purchase price when the transaction goes forward and it is forfeited when the transaction falls through, by reason of the fault or failure of the purchaser.
4) The legal position regarding earnest money or advance is that
a) It must be given at the moment at which the contract is concluded.
b) It represents a guarantee that the contract will be fulfilled or, in other words, "earnest" is given to bind the contract.
c) It is part of the purchase price when the transaction is carried out.
d) It is forfeited when the transaction falls through by reason of the default or failure of the purchaser.
5. Unless there is anything to the contrary in the terms of the contract, on default committed by the buyer, the seller is entitled to forfeit the earnest
6) Case law in support of your query Supreme Court of India, Satish Batra vs Sudhir Rawal ,18 October, 2012.
7) You have the option to retain the entire advance money based on above legal position or as a good will gesture you can choose to retain 10% of advance monies and refund the balance of monies to the defaulting purchaser.
Hope this information is useful