Buyer is in breach

I sold a property that offers accommodation to students. Buyer made a down payment was on 29 July 2019 and the balance was to be paid on 20 November 2019. The other condition was that I as seller should vacate premises by 9 August 2019. A contract of sale was not made but an acknowledgent of receipt of the down payment for the sale of the property and the conditions stated as above. Buyer has failed to honor the agreement on the promised date. After the breach date, buyer paid some money but he has not fully paid till date. Buyer collected rentals for 1 semester last year. In December 2019, I proposed that I start collecting rentals but buyer wrote me a letter in January this year telling me that he has no objection for me to be collecting rentals as long as he will deduct all rentals from what he owes me. Due to breach of agreement on the part of the buyer 1. Do I have the right to claim the rentals the buyer collected before breach? 2. If i cancel the agreement but don't find an alternative buyer in good time, how much time iam I given as a seller to refund the earnest money and the other he paid after breach?