reverse charge /output charge

If i am correctly considering your Question:

You have unregistered purchase of Rs.1,50,000 on which you will be liable to pay tax @ 18% which amounts to Rs.27000

Now you make a supply of Rs.1,00,000 hence your output liability is Rs18000 

But since you have already paid tax on purchase and your credit is Rs.27000 which is enough to adjust your output liability of Rs.18000 hence now no amount is payable