Where a memorandum of any agreement the registration of which is required by section 28, is not sent to the Commissioner as required by that section, the employer shall be liable to pay the full amount of compensation which he is liable to pay under the provisions of this Act, and notwithstanding an thing contained in the proviso to sub-section (1) of section 4, shall not, unless the Commissioner otherwise directs, be entitled to deduct more than half of any amount paid to the ¹[employee] by way of compensation whether under the agreement or otherwise.