A very very difficult one this one. On the one hand you obviously don’t want to lose the work but then on the other hand you shouldn’t be having money still owing when a client has received the final version of their site.
I always have a contract which fully states before even getting into what they want their site to do which states 50% up front and the final 50% upon completion of site before it goes live.
Perhaps you could write up a contract which breaks down the site into chunks and then they pay for the site in chunks?
Other than that I’m not too sure what to say, I’m sure others will have come across this before though and will be able to lend a hand though.
I sometimes do wonder if some clients have a clue about anything or whether they are just trying it on sometimes because if some money is left over then they are going to find things that they want to change once the site is live and you’ll probably never see that money as they will believe their site isn’t finalised yet.
It’s a bit like going into a car showroom and paying for half a car and saying you’ll give the rest to the dealer after you’ve ridden the car for a bit!
Wishing you the best on whatever you decide to do here. Interested to see how it all goes for you.