query.py

from django.db import models


q = (
    Lancamento
    .objects
    .annotate(
        val=models.Case(
            When(account_from=8, then=models.F('valor') * -1), 
            default_value=models.F('valor'), 
            output_field=models.DecimalField()
        )
    )
    .filter(
        models.Q(account_to=8) | models.Q(account_from=8)
    )
    .values('account_from', 'account_to', 'val')
)