I'm going to lie to my best friends after promising to tell the truth
Would you like to tell me why you are going to lie to your best friend?
I don't want to tell the truth.
You should keep your promises and it's wrong to lie when you said you would tell the truth. Lying to your friend can damage your relationship if they ever find out.
It's not that big of a deal and it's a white lie so they won't care.
I think they might since you promised not to lie. Now they will know not to believe in your promises either.