Shulchan Aruch of Rabbi Shneur Zalman of Liadi, Volume 12: Choshen Mishpat

In this and the following instance, the person would in fact be willing to carry out the favor. Nevertheless, since his present circumstances prevent him from doing so, he should not make a gesture that will cause his friend to be grateful to him when in fact there is no reason for him to be.

the barrel was already open, but the host made it appear that he is now opening it in honor of his guest.

the manner in which the rebuke is administered is important. See

Now, once a person makes a pledge to give a present to a poor person, by contrast, he is bound by it and it cannot be retracted. Seemingly, since giving the Levites tithes is a mitzvah like giving charity to the poor, such a promise should also be binding.

Ahavas Chesed (2:3; 5:3; 6:9) writes that even a person who derives his livelihood from lending must give an interest free loan if approached by a Jewish borrower who needs money to buy his basic necessities. Indeed, giving such a loan takes precedence over the lender’s desire to generate additional income to provide for a more lavish lifestyle.

It is forbidden to mix a small amount of [inferior] produce into a large amount of superior produce so that [the inferior produce] will not be recognized95 and [then] sell the entire quantity under the presumption that it is [all] of higher quality.96

Nevertheless, [the lender] must return every implement [taken as security] to the borrower when he requires it and come back and take it when he does not require it.76

As stated there (ibid.:11), the obligation to return the article or the money applies only when both parties are Jewish. Although it is forbidden to deceive a gentile regarding the value of merchandise, it is not forbidden to take advantage of an error the gentile makes on his own.

who speaks most disparagingly of one who seeks to profit by deceiving gentiles. He concludes stating that Divine Providence will not bless such a person’s endeavors. Even if he may profit in the short term, his money will not be graced with blessing and ultimately, will be lost.