People who want to buy cars through the bank or through leasing schemes ask their employers for a letter stating their salary. Is it permissible for the person whose job it is to issue such letters to give them to them or not? 

Praise be to Allaah. 

We put this question to Shaykh Muhammad ibn Saalih al-‘Uthaymeen, may
Allaah preserve him, who answered as follows:

If he knows that they want that to help them engage in this haraam transaction,
then it is haraam, because the Prophet SAWS (peace and blessings of
Allaah be upon him) cursed the one who consumes ribaa, the one who gives
it, the two who witness it and the one who records it. And Allaah has
forbidden this when He said (interpretation of the meaning):

“…
do not help one another
in sin and transgression…” [al-Maa’idah 5:2]

  And Allaah knows best.