You miss the point - the client does not have to pay anything at all
By using an invoice, you the producer, clearly indicate that the project will not start until the initial payment is received - i.e. no pay no music made
No final product is delivered until the final payment is made - so the producer is only ever going to lose half of the fee not all of it
If the client chooses to runoff with the (sub-
standard 64kbpscbr mono) mp3 used for demoing the work without paying the final fee then they have got their moneys worth and that is about it
at all points in the process, unless the work is done as a work for hire (different scale of payments btw), ongoing logging of copyrights and final registrations should be done - this will stop the client from claiming "all my own work" or at least give the producer grounds to pursue litigation
if necessary, i.e. in spite of shit quality the track blows up and money is flowing like beer at a brewery party