Converting full adder to subtractor using inverter
Inverting all bits gives you the ones complement and I assume you want to work in twos-complement format.

To make a number negative in twos-complement format you can do a ones-complement (invert all bits) followed by addition of 1. 

Thus you can convert an adder to a subtractor using the two's complement but that requires another adder :-). 

However there is a trick you can use sometimes: if your adder has an unused carry-in port you can use that to perform the extra "add-one".