[Nyororin]

After about 5 minutes of brainstorming with my husband... (Whose first question, by the way, was "Is the person saying it a native Japanese speaker? Or just some kid who knows a few words?") The only things we could come up with for use in parting were "それじゃ" or "それでは" - which could be shortened to そいじゃ and そいで.

Unless the context is totally different and it`s just being linked to bye because it sounded close to さようなら, it`s is most likely one of those.