In many circumstances, yes. However, I would like to point out a few differences between the words: (1) Payback *can* have a very negative meaning, and in particular the meaning/overtone of "revenge." So we'd say "It's payback time" when we're about to do something to make people "pay" for the *bad* things that those people did to us. (2) Repayment, on the other hand, is generally neutral and is used most often with money. For example, we can say "the repayment of a $10,000 debt is a difficult task for John, who is unemployed now." In (1), you will definitely not say "it's repayment time." However, in (2), you can say (although I prefer "repayment"), "the payback of a $10,000 debt is..." I hope this is helpful.

I'd have to agree. They usually are not used interchangably. Usually when we say "payback" it has a negative meaning, meaning revenge. Example: "The murder last night was payback for a drug deal gone bad." However, we also use a phrase, "to pay sb back," to mean to return somebody's money to them. Repayment is usually neutral. "The repayment of a debt is difficult when you don't have a job." The verb form is repay - "I don't think I could ever repay your generosity."

No. When I borrow money, I always replay the loan promptly. Rude people look for "payback" when they perceive that they have been slighted. I can pay back a loan though repay is a better choice. Kindness can be repaid with kindness. Note that "pay back" is substandard.