Bava Metzia 87a
גדול שלום שאפילו הקב"ה שינה- ואדוני זקן/ואני זקנתי
The Mogen Avraham 157 quotes The Sefer Chasidim
who writes when Chazal said “you may say non truths
for the sake of peace, " it only applies to the past and
not for the future.
The Sefer Chasidim explains this halacha :
Reuvain has the money but doesn’t want to lend it to Shimon because he doesn’t trust him.
1) Shimon asks Reuvain to lend him money. Reuvain may not lie and say he doesn’t have any money. (future)
2) Shimon asks Reuvain How come last week when I asked you for money, you didn’t lend me. Reuvain may answer falsely and say "I didn’t have any money." (past)
Reuvain may do so because if he lies, there will be peace between them but If he says the truth (I don’t trust you) then Shimon will be upset & angry.
The obvious question is:
If you may lie for the sake of peace , why should there be a difference between past & future?
Mori V'Rabi Harav Hagaon Reb Shlomo Miller Shlita explained the reason for the difference:
"In case # 2) there are only two options
a) lie – will be peace
b) truth – will be upset & angry, therefore you may lie for peace.
In case #1) there are three options
a) lie,(don't have)
b) truth,(don't trust you)
c) lend him the money.
The only time that you may lie is when lying is the only option for peace.
When you have another option to keep the peace (lend the money)then you may not lie. You may lie only when the only alternative for peace is to lie".