Is it permissible for an employee to accept a per diem allowance (for work outside the city) when participating in festivals and seminars within his city, on the pretext that there is no specific allowance for these activities, given his knowledge that not accepting these activities will cause him harm?

If attending festivals and seminars is your right, and you cannot obtain it except by signing the secondment, then there is no sin in that, even if it involves lying. Lying is permissible if it is a means to attain a right, as mentioned by Ibn Al-Qayyim and Ibn Al-Jawzi. It should be noted that attending festivals and seminars that involve reprehensible acts is forbidden unless there is a necessity or for the purpose of changing the reprehensible act.