This is where you got it wrong and he is blackmailing you over, and succeeding.
So again, the cheque is by law, a legal mean of payment. If he refuses to take the cheque, then it means he is refusing a perfectly legal payment. And in that case, he doesn't have the right to break the contract, let alone claim any sort of breach from your part.
Then again, if you're afraid that he reimburses you on the basis of 1500 in case of termination, (which only a judge can decide) then you understand well that anyone can perfectly pay his USD dues in Lebanese Pounds at the rate of 1500. Which is actually what the law stipulates.
In such case, just use that fact to your advantage. Go to the public notary, deposit your dues in Lebanese Pounds at the official rate, and notify him to come and cash them. By doing so, you would have legally fulfilled your obligations and he has no legal ground to claim otherwise. He can accept to cash them or not, but that wouldn't change a thing. You would still have legally fulfilled your obligations towards him, either way.
And if the whole amount that you are supposed to pay him until the end of the contract is not too big, then put all of it at the public notary at the rate of 1500 and notify him to cash it all, with the mention that you have fulfilled all your contractual obligations, absolving yourself from any liability, asking him to fulfill his obligations on due time.
Understand first that you need to stop listening to the seller and believing everything or anything he says. And stop calling him "the owner". He is not the owner. He is the seller and YOU are the owner now. The property is legally yours unless a judge rules out otherwise.
Bottom line is: If you fail to reach a fair compromise with the seller, along the lines and boundaries we talked about earlier, then you should immediately do all that is mentioned above, under the supervision of a lawyer, who will take appropriate legal measures to protect and enforce your rights.
You have the uncontested legal advantage and upper hand on all counts, and you should hence impose your fair solution on him, instead of submitting to his greedy and inconsiderate and unreasonable wishes and claims and demands.