daf bit: Bava Metzia 115

We previously (2 days ago) learned in the mishna: if a man lends money to his fellow he may take a pledge (collateral), but only through the court and there are restrictions. If the debtor possessed two articles, the lender may take one and must leave one. He must return the pillow by night and the plough by day. However, if the debtor dies, he does not need to return the pledge to his heirs. The g'mara further expounds that the debtor retains his basic standard of living; a rich man must be allowed to keep the comforts he's used to.

Now on today's daf we learn: a man may not take a pledge from a widow at all, whether she is rich or poor. That is the mishna, but R' Shimon says in the g'mara that he can take a pledge from a rich widow but not a poor one. This is because you must return the poor person's pledge each night (for a garment). That's ok in the case of a male debtor, but if a man shows up at a widow's house every night to return her pledge, that looks bad in front of the neighbors, who won't know it's to return a pledge and just see a man visiting every night. Therefore we just don't allow taking the pledge in the first place for a poor widow, but a rich widow wouldn't need to have the pledge returned each night so that's ok. (113a, 115a)

The g'mara here does not consider changing circumstances. I speculate that R' Shimon would require the man to return the pledge (and then not retake it) if the rich widow had a change of circumstances.