Christ also sais this: http://biblehub.com/matthew/5-26.htm
Yes, but in context that was part of a parable of reconciling with your brother rather than being thrown in prison. It's not really applicable to salvation.
Nor is it applicable to sin. For if we must pay the "full penny" for sin", then we must suffer the second death and Christ's sacrifice is of no effect.
It's just good advice to make good on your debts and don't get thrown into prison.