Assalamu alaikum.I was having a debate with a friend through email about two issues.I made a condition that I will reply to the mail only if both issues are discussed in the same mail.But accidently I broke the condition I made when I replied to his mail which touched upon one issue only.Does this come under breaking of oath or vow?Do I have to make an expiation?Thank you

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

What you mentioned does not apply to a vow or an oath, because a vow does not take place except with absolute words like saying: “I have to offer to Allaah such and such if.....”; similarly, an oath does not take place except with words of an oath, such as swearing by Allaah, or by His Names and Attributes, but being determined to do something, or saying “I’ll do something” and similar expressions that do not render a person under the absolute obligation to do something, then this is neither a vow nor an oath, and a person is not religiously obliged to do anything as a result. 

For more benefit, please refer to Fataawa 91107 and 90339.