Is it considered a sin to sell a house that a father gifted to his son — with the son contributing a portion of the money to its construction — and then distribute its price among the heirs after the father's death to pay off his debt and distribute unequal amounts among them?

Scholars have differed regarding the obligation of equitable distribution among children in gifts. The preponderant opinion here is that the gift is void, and the wealth reverts to an inheritance to be divided among the heirs according to Sharia after the fulfillment of all obligatory rights, unless one of them waives their share. As for what you contributed to the construction of the house, if it was on a voluntary basis, you have no right to reclaim it. However, if it was with the intention of being repaid, you have the right to claim it if you can prove it, though it is preferable not to demand it from your father.