Is it permissible for me to take an additional 30,0 when lending my friend 150,0 to furnish and rent out his house, on the pretext that my money was instrumental in completing the furnishing and renting of the house?

The described transaction is purely a loan from you to your friend, and his promise to repay more than the principal amount is usury (riba) by the consensus of scholars. His statement that you are the cause holds no weight. It has been authentically reported that the predecessors (Salaf) forbade accepting a gift from a borrower, even without a condition. How much more so then, when it is stipulated at the time of the loan or promised beforehand.

To correct the transaction, your friend should make you a partner in the apartment with this amount, in exchange for a percentage of ownership in the apartment. You would then receive a share of the rent proportionate to your ownership in the apartment, just as you would receive a share of its value if it were sold.

However, merely giving him the amount for him to complete his apartment and then repaying it to you with an increase—this is usury.