Contract to sell

Dear PAO,

I have a real property located in Rizal. This was supposed to be our rest house. Due to the coronavirus pandemic, I was constrained to sell my property to my best friend Miko. He told me he could not pay the purchase price in full, so he made a down payment last March 2020 and promised to pay the balance after four months. My real estate broker, who is also my friend, advised me to have a contract to sell with a condition that a deed of sale would be made after full payment. Months have passed, but Miko failed to pay the balance. I tried to sell the property to another prospective buyer, but Miko warned me that I can no longer do the same considering that we executed a conditional sale. I am confused right now, can you enlighten me on whether I can sell the property to another buyer?