Working for companies whose wealth is mixed is makrooh, according to the majority of scholars, but it is not haraam unless the work itself is haraam or involves helping others in doing things that are haraam, such as working in the riba-based bank itself.
Makrooh does not mean haraam, so the one who does that is not sinning, but it is better for the Muslim not to do that and to avoid it, and to look for work that is permissible.
Selling by instalments is permissible, but two points should be noted:
1-The contract should be free of clauses imposing a penalty in the event of late payment, because this is stipulating riba, and it is not permissible to write such a clause or sign for it.
2-The instalment plan should be made directly by the company that owns the product, with no involvement of any riba-based bank. That is because the involvement of the riba-based bank in this transaction means that it will pay the whole amount on behalf of the purchaser, then it will receive the instalments, so in fact this contract is a riba-based loan. The bank will lend the agent one hundred, for example, and the agent pays it on behalf of the bank to the seller, then it will take one hundred and twenty in instalments. This is forbidden riba, as is obvious.
If the company is selling the land in a permissible manner – without the intervention of a riba-based bank – then your work is permissible and the salary that you get is permissible. But it is better for you to look for purely halaal work, as stated above.
If the company is selling it in a haraam manner, such as if it stipulates late-payment penalties, or the bank is involved in the instalment payments, then we should look at the nature of your work in this case. If it involves helping in this haraam transaction, such as writing or recording or checking etc, then your work is haraam, and the salary that you get from it is haraam, but what you took before you found out that it is haraam is yours to keep and you do not have to give it in charity.
Whatever you got after you found out that it is haraam, you should get rid of it by giving it in charity, and you must also leave this job.
If the job you do is partly permissible and partly haraam, then you should strive to work out how much of it is haraam and get rid of the corresponding amount of your salary. See question no. 81915.
If your work does not involve helping others to do haraam, rather you work in a department that has nothing to do with sales by instalment for example, then we hope that there is no sin on you for this job,
We ask Allaah to guide you.
And Allaah knows best.