What is the ruling on obliging the buyer to pay the remaining amount for the building, after it became clear that there is a judicial lien on it for $20,0, knowing that the remaining amount due to me from him is $40,0?

If the term in an installment sale contract is unknown, such as stipulating that the remaining amount be paid upon demand, then the contract is invalid according to the majority of jurists. This is because it leads to gharar (excessive uncertainty) and disputes.

As for the building being mortgaged, selling a mortgaged item is valid but contingent upon the approval of the mortgagee. If the mortgagee approves the sale, it becomes effective. Otherwise, the buyer has the option to either annul the contract and reclaim his money, or wait for the mortgage to be released and then pay the price.