What is the ruling on taking a sum of money from one's father to buy land and build a house, then giving him back a portion of the amount, and the bank subsequently buying the house from the father and selling it to the son to benefit from the state's subsidy on the profit margin? And what is the ruling on buying through a bank in general?

The described transaction is a forbidden stratagem and constitutes usury (riba) because the house is, in reality, your property. Thus, the transaction devolves into the bank giving you a sum of money and recouping it with an increase. It is impermissible to transfer the contract concluded between the client and the issuer to the institution. Any previous contractual relationship between the client, who orders the purchase, and the original seller, must be canceled, so that the transaction does not become merely a usurious loan. Indeed, the absence of a contractual relationship between them is a condition for the validity of the institution's execution of the murabaha operation for the ordering client. There is no harm in dealing with an Islamic bank if it has a trustworthy Sharia supervisory board, by it purchasing a house or a car for you.