Prepayment before home loan disbursement

Hi, I'm a salaried employee working in IT sector. Planning to buy a under construction new flat from builder valuation of around 28 lakhs. Upfront while booking i had to pay 20% as down-payment and planned to arrange rest of the money via home loan. Now as per agreement, I have to pay 5% of remaining amount (after down-payment) at the time of roof casting of each floor and 25% at the time of brick work of said flat. I've applied for Home loan in HDFC Bank and SBI. Pre-sanction letter has been generated but legal evaluation step is pending in both cases. As per bank, It'll take another 1 month time. But builder is constantly asking for money as roof casting of ground floor has been completed and roof casting of 1st floor is scheduled to be done in next 10 days. He is also pressuring me to take hime loan from his partner bank so that he can get the disbursement in 1 week but rate of interest is higher in his partner bank. I'm not in position to deny as i do owe him money as per the agreement done. Under such circumstances, only solution I can offer is to pay 1st 5-10% money from my contingency fund. But I'm confused if that'll cause any issue. Because in the agreement I've signed It was written that 20% paid by cheque and rest of the money to be paid by Home loan. Will it be an issue afterwards or do I need to execute the agreement again if I make extra 10% payment from my end contrary to the agreement (as now it'll become 30% down-payment and 70% home loan). What may be the consequences. Can anyone please advise? Thanks in advance.