Regarding Flat cancellation

I booked two flats at builder on condition that if fund of one flat is not possible out of two within period of seven months from the date of booking, then payment made to both flat will be transferred to one flat with out any deduction .this was agreed between sales person of builder and me before booking.I asked for written document for same she denied saying she will be there to get it transfer and gave me promises verbally .By the time of Six month I paid 20 % of total consideration to both flat and later payment to one flat is not possible due current market condition .Between builder has send me email notice of termination of both flat reason ,delay in further payment . Now when I went to builder for doing so as agreed above condition between sales person and me.Builder is saying the 20% of total consideration of flat will deducted .Builder maintenance cost sheet where it is mentioned as booking amount will not be refundable .Agreement is not registered yet .I am repeatedly sending emails of praying to transfer the amount to other flat as it was agreed .But no reply from them.When I talk to sales person she saying the finance department is not accepting it though she is accepting the above condition was made before booking. I don't have any written document for that but audio and phone call recording where she is clearing accepting the promise made and some support of whatapp chat. please suggest me what to do