Is the amount offered as a "gift" or "aid" by a supplier who has overdue financial entitlements, in exchange for receiving his due payments, considered a bribe or a permissible (halal) form of assistance, especially if the supplier agreed to payment terms that suit the company's situation?

This is a bribe, and it is one of the major sins. The Messenger of Allah, may Allah bless him and grant him peace, cursed the briber and the bribed. Al-Hasan and Sa'id ibn Jubayr interpreted "Al-Suḥt" in the Qur'an as bribery. Paying and accepting a bribe is unlawful, unless it is for attaining a right or repelling injustice, in which case it is permissible to pay it, and the sin is on the one who takes the bribe. The sin falls upon you if you accept this bribe, and you must return what you have taken and deliver the right to its owner, because delaying the right of the wealthy is injustice. So, beware of consuming unlawful wealth, for every body nourished by unlawful means, the Fire is more befitting for it.