We have a tenant in a condo we are renting who is leaving

We have a tenant in a condo we are renting who is leaving June 30. The condo is also listed for sale as of April 28. Condos in the building take a long time to sell even though it is a seller's market. We could not afford to have it sit empty so we rented the condo out to a new tenant on a six month fixed term lease from July 1 to December 31. The new tenant was aware that the unit is also for sale and for this inconvenience we rented it to him at a lower than market rate for the 6 months. A unit very similar ours mine just sold after being on the market 8 months. Suddenly, we got a full price offer and the new owner wants to move in right away. We spoke to the tenant who is scheduled to move in July 1 and offered him 2 months rent to cancel our lease agreement. He says he is not willing to agree to anything until he talks to his sources. We have not accepted our full price offer yet as we need to sort this out. We also have a cheque from the tenant for the damage deposit that is dated July 1 so it has not been cashed. We were wondering if there is any other way around this.