Willing a Universal Law Form of the Categorical Imperative: "Act only according to that maxim whereby you can at the same time will that it should become a universal law. (Kant in Solomon and Martin, 290 and in Solomon and Greene, 279)."

Kant's second example about keeping promises (the lying promise) involves borrowing money and promising to pay it back even though one knows one can never do so (Kant in Solomon and Martin, 291 )

Maxim: "When I believe myself to be in need of money, I will borrow money and promise to pay it back, although I know that I never can do so (Kant in Solomon and Martin, 291 )."

Universalized Maxim: Whenever anyone is in want of money, they will borrow money and promise to repay it, although they never can do so. Or, more generally, ". ...anyone believing himself to be in difficulty could promise whatever he pleases with the intention of not keeping it...(Kant in Solomon and Martin, 291)".

Not Rational: The promise itself would become impossible. Promising and truth-telling would become meaningless. The universalized maxim is logically inconsistent, self-defeating. Note: For Kant the negative consequences are not relevant.

Perfect Duty: Therefore, one has a perfect, rigid duty not to make a lying promise.

Parallel Examples - Examples that create a perfect duty such as stealing, cheating, etc. 

Willing a Universal Law Form of the Categorical Imperative: "Act only according to that maxim whereby you can at the same time will that it should become a universal law. (Kant in Solomon and Martin, 290 )."

Kant’s Fourth Example: "A fourth man finds things going well for himself but sees others (whom he could help) struggling with great hardships; and he thinks, what does it matter to me? Let everybody be as happy as Heaven wills or as he can make himself; I shall take noting from him nor even envy him; but I have no desire to contribute anything to his well-being or to his assistance when in need. (Kant in Solomon and Martin, 292 )."

Maxim: I, a prosperous person, will not help those in need.

Universalized Maxim: No prosperous person will help those in need. The maxim can be universalized and still be possible unlike the maxim in the lying promise example.

Will contradicts itself. Wills people be helped, if oneself; wills people not be helped, if others. Positions not reversible. "For a will which resolved in this way would contradict itself, inasmuch as cases might often arise in which one would have need of the love and sympathy of others and in which he would deprive himself, by such a law of nature springing from his own will, of all hope of the aid he wants for himself (Kant in Solomon and Martin, 292 )."

Imperfect Duty: Therefore, the person who is well-off has an imperfect, laxer, meritorious duty to help those in need. (Perfect duties are when the universalized maxim itself is logically inconsistent as in the lying promise example.)

Parallel examples: slavery, discrimination, an injured person who needs help from a healthy person, etc.

Immanuel Kant, Grounding for the Metaphysics of Morals. 1785.  Excerpts in Morality and the Good Life: An Introduction to Ethics through the Classical Sources. 4th ed. Eds. Robert C. Solomon and Clancy W. Martin. Trans. James W. Ellington. Boston: McGraw-Hill, 2004. 262-311.