Is it permissible to take a sum of money from someone in exchange for giving them 20% of it and repaying it later with checks, due to a need for the money for life's obligations?

If the question implies that you would pay a percentage of the loan to someone who facilitates the loan for you, this is a ju'alah (commission/reward) for a permissible act. The majority of scholars stipulate that the ju'al (reward) must be known. However, some scholars have permitted it if the uncertainty does not prevent delivery, such as someone saying: "Whoever demolishes a fortress shall have one-third of what is inside it." But if the loan is from the borrower himself, then he is not entitled to anything, because taking interest in that case would be usury (riba), not a commission (ju'alah).