On the previous daf the Gemara quotes a baraita that instructs the victim of a robber or of someone who paid forbidden interest on a loan to forgo repayment for his loss, even if the individual who stole his money wants to give it back to him. The reasoning behind this ruling is the desire to encourage a sinner to repent, and the fear that forcing him to repay all of his ill-gotten gains will discourage him from repenting. It should be noted that this is only if the individual steps forward and expresses a desire to repent. Someone who has been wronged can certainly approach the courts and demand justice.
The Gemara on our daf suggests that we must distinguish between a situation where the stolen item still exists ? where it can be accepted in return – and where it no longer exists – where we will discourage the victim from demanding its return.
One case that appears to run counter to this argument is the case of a marish ha-gazul – a beam that has been built into a building. In that case, although the beam is still extant, we do not require that the beam be returned – only its value is paid. The Gemara explains that since the beam has been built into a structure, and removing it would cause a serious loss, we view the beam as if it had been destroyed, and only require payment.
What is clear from this ruling is that takkanat marish (or takkanat haShavim) may free the robber of the obligation to return the original object, but it does not free him of his obligation to pay. The Ra’avad and the Rashba explain that there are two separate situations of takkanat haShavim. In one, if the object is still extant, but returning it would cause a significant loss, the robber will only have to return the value of the object. The second takkana applies in a case where the stolen object no longer exists. In that situation, if the robber wants to do teshuva, then the victim should not accept repayment at all.