when your relative buys a car and asks you to complete the payment on his behalf, this is regarded as a loan. It is up to you to lend him the amount or not but it is impermissible that you receive any interest on the loan. This is because doing so is considered Riba (usury).
(Part No. 14; Page No. 130)
Charging an amount of money in return for concluding a sale contract or mediating between two parties is permissible subject to the agreement. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.