Sales agreemnt- buyer did not pay amount in due date

Dear Sir,

We entered into a house sale agreement with a buyer, initially they have paid 25% and entered into agreement saying they will be paying remaing amount in next 5 months. The agreedate is about to expire in few days, now buyer want to have some more time( in months) to complte the acitivity, which is not agreeable to us. Please advice how do procede to avoid further complications.

Thanks,