What is the ruling on paying a sum of money or giving a gift to a purchasing manager to secure a purchase from him? And is it permissible to offer deals and gifts to the buyer (the entity) in exchange for a purchase?

It is not permissible for an employee to accept a gift because of their position unless authorized by their employer, as this is considered among the gifts given to workers, which are deemed ill-gotten gains (ghulul). If the employee stipulates a gift for fulfilling people's needs, then this is ill-gotten wealth (suht) and consuming people's money unjustly. It is not permissible to assist them in this by offering a gift or money directly, or by providing a service from which they benefit, as long as they take it without the permission of their employer.