<human>: Can you summarize the given text in a few sentences?

Hi all, 

I'm a long time lurker on this sub and now hoping for some advice myself. 

My father in law loaned my husband and I a substantial sum of money to help us buy our house 18mths ago. 

We discussed the arrangement extensively and reached a verbal agreement that the money would be payable, including interest, on sale of the house or taken from my husbands inheritance, whichever is sooner. He (my FIL) got his lawyer to draft an agreement which went back and forth because I am also a lawyer and wasn't happy with the drafting (too much ambiguity). We never actually finalised it and have been going along without issue until this week...

My FIL is now demanding we pay a monthly interest amount. This was never part of the original agreement. He was very nasty to my husband about it when he blindsided him with the request last night. Name calling, aggression, questioning out financial decisions etc

I don't know what to do. I am confident that legally he would be stuffed in trying to come after the money, but my husband wants to find a way to pay it. I think this is because he doesn't want this to ruin what he has left of their relationship. 

I totally understand this and would probably be ok with it except we have a baby due in two months and money will be tight for a while. 

I guess I want to know how I can deal with this sensitively but in a way that doesn't put my husband and I under huge financial pressure??
<bot>: father in law lent us money and suddenly decided we have to pay him interest. Has been nasty and aggressive and don't know what to do.