Is there a word or phrase for "promises that can't be kept"?

I know that renege is a word that could suit in here. But as I understand, 'renege' describes the failure to keep a promise.

But, sometimes, we make promises that we know all too well that it can't be kept for long. Is there any word or a phrase for such promises?.

You are describing an empty promise, which is a promise that will not be kept. It doesn't directly describe the timeframe in which the promise is broken, but an empty promise will usually not be fulfilled at all.

In the movie Mary Poppins, Mary refers to such promises as "Pie crust promises". "Easily made, easily broken".

Michael: Will you stay if we promise to be good?

Mary Poppins: Och! That's a piecrust promise. Easily made, easily broken.

false promise

A promise that is made with no intention of carrying it out and especially with intent to deceive or defraud m-w

Now it might be said that the moral rigorism involved in the view that we ought never, in any circumstances, to make false promises, tell lies or break promises is just a personal idiosyncrasy of Kant's, and that we do not have to treat it as an integral part of his moral philosophy...It is one thing to argue that if everyone made false promises whenever they thought it to their advantage there could be no such thing as a promise... ref.

A promise is false, not by virtue of not making promise, but by virtue of not doing the relevant things subsequently. Hence a false promise is a case of a promise. ref.

The gravamen of Ms. Tarmann's complaint was that the defendant insurer made a false promise to pay for car repairs upon their completion a future event. ref.

There is a bad faith promise in law:

A bad faith promise remains an effective promise “but it is not a lie or a misstatement.” ref.

If you make a promise you know you cannot keep, the word for that is lie.

I'm not entirely satisfied with this because lie is a broader category; not every lie is a promise, other than in the guarantee (a fact) nuance of promise which appears in I promise you that this is true.

However, it's usually clear from context that a reference to some lie is actually about a bad faith promise, rather than some other lie, such as a cover-up of events or failed responsibilities.

I suspect that there might not exist a single verb which we can fill in for

Bob ____ed that he will return the money

where where ____ed specifically means lied as he promised.

However, note that in an example like this we do not need such a verb, because the complement "that he will return the money" establishes the context that a promise is being made, allowing us to just use the verb to lie:

Bob lied that he will return the money.

Somebody who commits or attempts to do something they can't succeed at can be said to have bitten off more than they can chew.