If I work for a Shaliach that is supposed to pay for my car, am I allowed to use my Maaser money to pay for it and consider it as if I gave that Maaser to the Shliach?

If the car is being used primarily for Shlichus work, you may pay the cost from Maaser. The car remains the property of Tzedaka and does not transfer into your possession even if the Shlichus no longer requires a car. Obviously, this does not mean you can buy/lease a luxury car, rather it should be an average car.

However, you may not use Maaser money for a car used primarily for household purposes, even if when taking the job the Shaliach added a car to the contract, which means that the Shaliach owes you a car and you wish to absolve him from the debt based on the poor financial state of the Mosad. Although there are Poskim that say you can do so under certain circumstances, the majority of the Poskim are of the opinion that it cannot be done retroactively. However, if this was your original intention it is OK. Nonetheless, one must inform them the debt was canceled. Also, some poskim recommend if possible informing the recipient that it came from Maaser.

What you can do is as follows: give the money for the car to the Shaliach for him to pay his debts. The Shaliach can then give you the money towards a car. One may not stipulate with the Shaliach that he will return the money to you. If the Shaliach decides to use this for other debts he may do so. As a general rule, if its a loan which can reasonably never be retrieved, it is no longer considered Tzedaka.

It is also important to make sure that some of your maaser goes to actual poor people not just Torah institutions.

A side note: it might be smarter to first get paid from the Shaliach for the car. After paying him, you should consider whether to return the money as a donation. This way later the Shaliach does not automatically assume you are taking responsibility for it. You will also be actively giving Tzedakah rather then passively. It should be noted that many advise not to use Maaser money when it comes to employer-employee relationships, ensuring there is a healthy understanding between the parties that the employer assumes complete responsibility for paying his employees.

Sources:

ניכוי חוב – ראה רמא סי’ רנז סע’ ה, השך ס”ק יב, והפ”ת (ס”ק ה מסי’ רמט ס”ק א) מביא דעת הנוב”י תנינא יו״ד קצט. לבושי שרד בסו״ס תבו״ש קד. ביאורים שם ריג. אג״מ יו״ד א, קנג. מנח״ ה, לד, ח. תשובות והנהגות ח”ב סי’ תעא, צדקה ומשפט פ”ה הע’ נ, שו”ת משיב דבר יו”ד סי’ מט. ובתו”מ חכ”א ע’ 234 הע’ 183.

סוג הרכב – ראה בשו”ת מנחת יצחק ח”ה סי’ לד אות ו דרק דבר הרגיל יכול לנכות מהחשבון של מעשר ולא יותר מהרגיל וכל שכן מהמעות מעשר עצמו.

להודיעו – שלא יצטרך כפרה. וע״ד נתכוון לאכול בשר חזיר ועלה בידו בשר טלה.