R.J.
This is honestly tricky.
IF you can manage to convince your husband that your dad owes you big time and to consider this a down payment in payback...and that none of the 'strings' that come along with accepting 'money from family' -like the idea that they then have a right to x,y,z (aka the string will be there, but you're going to cut them...GO FOR IT!!! :) Durn straight.
HOWEVER...
I received some very good advice a long time ago: "If you're going to kick a man in the balls, physically or metaphorically, you'd better be planning on killing him next."
If you can't convince your DH that your dad owes you far far more than a discount he already would be receiving...pay up. It's not worth your DH's self respect, if that is what would be given in trade. Not to keep your father from being able to be snide, but to allow your husband to keep his manhood. Never emasculate any man that you don't despise.
With fingers crossed for you! Hope you can get that discount :)
R