sec 44ADA presumptive taxation

As per section 44ADA presumptive taxation, only 50% of the gross total receipts is taxable when total grid receipts are less than Rs 50 lakhs. Does that mean balance 50% which is not taxable has to be mandatorily spent by the assessee? What if the assessee is not able to spent entire 50% and he still taking benefit of presumptive taxation section 44ADA benefit? Will that unspent amount will be liable to tax again?