Some more info is needed regarding your relationship with the new homeowner. i.e. what you were hired to do, what was done and what is left undone. Your contract with the previous owner does not pass over to the new owner, that is for sure. You could come up with an agreed upon amount and give it to him. Chances are whoever he finds to "fix it up" will have no clue what your program was accomplishing in regards to your modules and will either end up making things worse or telling him they have to start from scratch anyway. At which point, you will probably be getting another call to come back.