Agreement of sale of property be registered with token amount payment by buyer

In property transaction involving resale of existing property below are major milestones, 1 token amount payment by buyer to seller 2 Registration of 'Agreement to Sale' 3 With copy of 'Agreement to Sale' and other documents buyer submits his home laon application with his bank 4 Payment of 'Own contribution' by buyer to seller in single or multiple payments. This can span over period. Minimum such period is time taken by his bank for approval of his home loan if allowed for loan approval (and not disbursement). 5 Approval of buyer's home loan by bank after all checks. Banks do take anywhere between 10 to 20 days for approval of home loan after property search and other formalities. 6 Submission of receipts by buyer with buyer's bank of all payments made by buyer to seller under 'own contribution' roughly 20% of agreement value 7 Issue of cheque by buyer's bank with amount matching with principal amount of seller's home loan account 8 Seller submitting the cheque received from buyer under #7 above with his home loan bank and closes his home loan account 9 Seller's bank returning all original property documents to seller 10 Seller handing over all original property documents to buyer 11 Buyer submitting all original property documents received from seller with his home loan bank 12 Buyer's home loan bank handing over photocopy of final disbursement of loan amount cheque 13 With photo copy of final disbursement cheque, buyer and seller registering 'Sale Deed' 15 Buyer submitting 'Sale Deed' original copy with his home loan bank to collect final disbursement of loan amount cheque 16 Buyer handing over final disbursement of loan amount cheque to seller and seller handign over property subject to clearance of cheque If buyer's bank requires proof (receipt) of his own contribution (payment to seller) only at the time first disbursement of home loan, buyer has liberty to use time period between 'Agreement to sale' and approval of his home loan by bank to make multiple payments to seller to complete his 'own contribution (20%)' By definition 'agreement of sale' is agreement of next steps to complete transactions between buyer and seller, the payment milestones can be during few days from the date 'agreement of sale' is registered. Question is - Is it must that 'agreement of sale' should be done only when buyer has paid considerable' amount' to seller? In extreme case why can't it be just with token amount?