How to ensure complete payment of property in a sale agreement

I am selling my property to an investor who's business is to buy and sell properties. It is a road Sula property so finally I have found a buyer who is ready to buy Only issue is that he wants to pay 10% during agreement of sale and rest of the amount after 4 months. Reason- As it is his business to buy and sell, he usually requests for 3 -4 months from all sellers ,so that he can find a buyer . If he doesn't find a buyer, he himself will take it on his name during sale agreement after 3-4 months. I have no problem waiting for 3-4 months. Only condition I have is that the 10% is non refundable amount. They have agreed to this condition and they will add this point to the agreement of sale. My question is, after 4 months if he does not pay me the remaining 90%, can I sell my property to some one else after signing agreement of sale? (not sale agreement). Because in agreement of sale, it will be written that he has 4 months time to pay me remaining 90%. Please advise me