Taijoku no Ori

Main character undertakes robbery in attempt to rub off the debt of a playmate, but during it lender suffers a serious injury. Our guy destroys evidence of debt and runs away, but eventually gets arrested by the police after his fellow's confession. Companion gets two and a half years of sentence, and protagonist gets seven years of sentence as main offender, but in a different prison. Such betrayal can not be pardoned, so an exquisite revenge plan is invented over these seven years.

[From vndbreview]