To pay guarantee using tithe money

Is one allowed to become a guarantor on someone else’s loan on a condition that if the guarantor be obligated to pay, he’ll use his tithe money for it?

If the borrower is someone who is fit to be supported by charity, for example, an indigent person or something similar to that, and the guarantor makes an explicit stipulation that he’s signing his guarantee with the intention of paying it with his tithe money, then he’s allowed to do that. But if he did not stipulate that ahead of time, then the answer depends.

If he is a guarantor who accepts responsibility as an arev kablan, that is, in a way that the lender is allowed to collect directly from him if he so wishes, without even trying to collect from the actual borrower, then he may not pay off this loan with his tithe money. The reason for that is that this is considered as if he were paying his own debt, and not the borrower’s debt.

If, on the other hand, he is a regular guarantor, he is allowed to pay off this debt from his tithe money, even if he didn’t stipulate this in advance. The reason for this is that, in order to collect from a regular guarantor, one first has to prove that the borrower has no money or property of any kind — and that is impossible to do. Therefore, this type of a guarantor is never actually obligated to pay. Because of this, he can pay off the loan with his tithe money, if he so chooses. (It’s important to know that when a guarantor writes 'אני ערב' — “I am a guarantor” that makes him only a regular guarantor, which means that the only way to force him to pay is to prove that the borrower has no money and no way of paying back the loan, which is almost impossible to do.)

Shulchan Oruch, section Choshen Mishpat, chapter 129; section Yore Deah, chapter 249