All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Your question included many issues.
Fist Issue: selling by instalments: we have already clarified that there is no harm in selling by instalments on the condition that the goods are sold with a definite price known to both parties and that this price should be fix and does not change even if there is a delay in the payments. With this fulfilled, there is no harm if the price of the goods sold by instalments is more than its cash price or that its price at a long term is more than its price at a short term; please refer to Fatwa 87403.
The Second Issue: if the customer rushes in paying the instalments during three months then the additional amount on cash payment for the goods will be dropped by 500 Rupees. This issue is known by the jurists by 'dropping an amount from the total price because of hasting in paying some of it cash'; it means conditioning dropping a portion of the debt in return for paying some of it cash. The majority of the jurists are of the view that it is forbidden as it involves Ribaa (usury and interest). Ibn Rushd said: 'The grounds of those who did not permit 'dropping an amount from the total price because of hasting in paying some of it cash' is due to its resemblance to interest [additional profit] if payment is delayed, and there is a consensus that this is forbidden; the resemblance is that there is a part of the price put in return for the time in both cases.'
The Third Issue: The company agreed with the customer on the rent according to the method mentioned in the question. This method is known by the jurists as rent that ends with ownership of the property. This method has many procedures, some of them are permissible and some of them are not permissible. The method mentioned in the question is among the types that are forbidden. For more details, please refer to Fatwa 90530.
Therefore, if you are able to involve yourself in what is permissible without engaging in what is forbidden or help others in such forbidden matters, then there is no harm for you in working in this company. However, if you are unable to limit yourself to what is permissible, then you are obliged to leave this work, and whoever leaves something for Allaah's Sake, Allaah will compensate him with something better.
Allaah Knows best.