Is there a sin in combining a pension and a salary from a job without insurance, and what is the ruling on the money earned from either of them?

What appears from the question is that the pension is a grant, and you resorted to deception against the granting authority to prevent it from being discontinued. This is not permissible. Therefore, since the condition for your entitlement to the pension has ceased, you are not allowed to take it. You must return what you took after obtaining the job, unless you informed the responsible authority about it and they gave you permission.