Is it permissible to take a sum of money from a relative to complete building a house, with the promise to repay it with a suitable increase "out of goodwill" without a prior condition, as a means of obtaining money?

As long as the borrower mentions an increase to the lender and knows that the loan will not be granted without it, this is neither a goodly loan nor a stratagem against usury; rather, it is outright usury. It is not a condition for usury that the increase be quantified or explicitly stated as a condition. Rather, merely mentioning it and the loan being contingent upon it is sufficient, and this is the meaning of stipulating it. The jurisprudential rule is that every loan that brings a benefit to the lender is usury.