Generally, when one has a debt and his creditor is mochel him from repayment, we view the mechilah as if he paid the debt. Consequently, we cannot prove that it is permitted to charge up to a sixth more from the fact that lashes are not administered. This is due to the fact that there is an owed payment – just that it was forgiven. It is possible, therefore, that while it is indeed forbidden to charge up to a sixth more, the reason that one does not receive lashes is because it falls into the category of a lav ha’nitein letashlumin – since he indeed owed the money.
The Shulchan Aruch (Choshen Mishpat 227:6) does not rule in favor of either opinion, stating rather that it is a safek whether one may charge up to a sixth more than the market value.
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