Of Course:
Seems like the parent is having a hard time financially, probably after taking a loan for the wedding and getting no Hakoras Hatov from the children (and not even offering to give the wedding presents to the parents). I would let it slide.
DY: J&W agrees that the money is owed. Then I am not arguing that you aren’t owed money. I am just saying you are going to have a hard time collecting since you didn’t make any agreements before hand.
That is similar to my argument before, which Rav Moshe seems to agree with. However, Rav Moshe in that teshuva seems to indicate that what is owed is the lowest common denomiator. Perhaps (maybe even probably) in a Din Torah “Of Course” could collect 100$, which he? agrees is acceptable in some cases.
I think what is bothering the OP is more the lack of Hakoras Hatov from the parents than the loss of actual money.