What should we do with a domestic cat we found in the street, after it became difficult to provide for its expensive special needs, and is it permissible to abandon it in the street if we cannot find someone to take care of it?

There is no harm in expelling a cat from the home, especially if its upkeep is costly, because sheltering it is not obligatory. There is no sin if it dies of hunger after being expelled, for whoever does what is permissible for him and damage results from it is not held liable. The sin falls upon the one who imprisoned the cat and did not provide for its needs until it died, as in the hadith of the woman who was tormented because of a cat she imprisoned.