I don’t understand what you are asking. You’re going to be buying a house, presumably one that is more expensive that will result in larger payments. You want to lower your payments, so you want to bring $150k extra to the table and reduce the loan amount from $400k to $250k, so the monthly would be lower….

…But you currently don’t have $150k, so you are hoping to tap into a HELOC to borrow $150k, until your parents/family sell a home in Europe, which you could then use to pay off the HELOC….

Question: if you’re buying a house with a loan, how do you plan on getting a HELOC at the same time on the same property? Also, if you borrow $400k, what is your LTV going to be? Is there going to be $150k worth of equity for you to tap?