Is saying "Insha'Allah" with the intention of not doing something, to avoid embarrassment, considered a sin?

You should not make promises or say what you do not intend to do, because saying "Insha'Allah (God willing)" is of no benefit if the person addressed believes that you are determined to act. Al-Awza'i said that whoever says "I will do such and such, Insha'Allah" while intending not to do it, has lied and broken a promise. It is better to remain silent or apologize for the promise instead of breaking it. As for what was said about the employee, if it was merely an internal thought, it is reprehensible ill-conjecture. If it was verbalized, it is backbiting, and you must seek forgiveness and repent to her.