Capital gain on devolopment agreement

hello all there is one case where development agreement was entered into with mr X in 2006. after making almost full payment of consideration mr x died in 2010. now to avoid legal dispute, builder make some payment to his legal heirs also in 2015. so my q is for computing capital gain , year of tranfer for legal heirs will be 2015 or 2006 in which original agreement was entered into. CII has changed drastically