If a father paid for his son’s house when the latter got married, and did this without stipulating that this money would ever need to be paid back, would there be any halachic basis for the father’s claim several decades later that the son must pay him back the amount the house cost? Thanks.
Under the circumstances (unless there are missing details), it would not be possible for a father to claim back the cost of the house.
Although we find in the Gemara and subsequently in halachah (see Choshen Miahpat 264:5) that a person who does a “favor” to his friend can later claim payment for it, this is only true where it is not clear that the “favor” was given as a gift.
In the case of a house for one’s son, the normal way of the world is that it is given as a gift, and that barring any stipulation, the father will not be able to claim it as his own or to claim payment for it. The passing of 30 years without saying anything certainly strengthens this assumption. Therefore the father will not be able to claim back the money.
Of course, the place to sort out monetary claims is in Beis Din, and the answer is given as advice alone.