What is the meaning of hawala transactions?

Taking ‘credit/refund of taxes’ which are actually not reached government is not uncommon in earlier VAT/Service Tax system. For Example: A sells to B (Tax amount Re.1, A goes missing and no tax paid to government), B sells to C (Tax amount Rs. 2, Paid to government Re. 1(after set off Rs.1), C sells to D (Tax amount Rs. 3, Paid to government Re. 1 (after set off Rs.2). Now D claims refund from government of Rs.3. While government has received only Rs. 2 i.e. Re.1 each from B & C. This is called hawala transactions/carousel fraud. By the time government can trace A, he went missing. Now under GST, since all these transactions shall be online and credit/refund shall be available only when tax has been received by government.