Nor say of anything, "I shall be sure to do so and so tomorrow"- Without adding, "So please God!" and call thy Lord to mind when thou forgettest, and say, "I hope that my Lord will guide me ever closer (even) than this to the right road." (18:23-24)
For me the crux of a lie is with respect to intent. Do you intend to deceive? Are you deliberately misspeaking? People say something in error, backtrack by saying "Oops, I lied," then say whatever it was they meant to say correctly. Did they lie? IMO, no, because they didn't intend to deceive. They only misspoke, which isn't the same thing as far as I'm concerned.
One thing I find of benefit in Islam is that we rely heavily upon the word "insha'allah," which means "If God wills" (or, in the verse above, "So please God!"). We use "insha'allah" for any future-tense sentence where something has not yet come to pass, thus the outcome is still uncertain. Thus, in a case like, "I tell my daughter that all will be well, that she'll have the boyfriend she wants", we would add "insha'allah." And then there is no lie.
(Of course, there are Muslims who abuse the "insha'allah," saying it when they have no intention of complying. "Will you meet me at the store by 5 o'clock?" "Insha'allah," and they don't show up. That's a lie.)
Now lazy record keeping is another thing, where "insha'allah" doesn't come into play, and there you're playing fast and loose with haram (forbidden) income. Best you keep on the straight and narrow there.