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The Gemora states that living rent free in a debtor’s house seems like interest, and is therefore forbidden.
Tosfos discusses the parameters of this prohibition. The Gemora states that living rent free is categorically prohibited, even if the debtor would have allowed the creditor to do so independent of the loan. Tosfos questions how a debtor can do any favors to his creditor, since these also would appear to be interest. Tosfos states that the prohibition only includes conspicuous activities, like living in someone’s house, but not things like renting out tools.
The Shach (Y”D 166:1) rules that any inconspicuous favors that the debtor would have done anyway for the creditor may be done. In addition, if they were known to all to be such close friends that they would have allowed each other to dwell rent free, this also may be done.
The Maharshal, however, states that any conspicuous favor may not be done, even if all knew that they would have done this favor without the loan in place.