Is taking a full month's salary and an end-of-service bonus for another month without working considered a suspicion of being unlawful (haram), knowing that I did not resign until they demanded it and refused my return to work?

If there is an agreement between the two parties, there is no objection to benefiting from what was given in exchange for waiving the right to the remaining period. The lease contract is binding on both parties and may not be terminated except with the consent of the other party. The tenant has the right to demand a sum of money in exchange for waiving the remaining period, as Sheikh Ibn 'Uthaymeen stated.