Can a Verbal Commitment Guarantee a Loan?  

Rav Yitzchok Grossman , Rosh Kollel of Greater Washington and Dayan at the Bais HaVaad

Question: When people cosign on a debt, the cosigner typically signs a contract to that effect. Occasionally, someone simply states verbally that he is guaranteeing a loan but he doesn’t actually sign on any document. 

Is a verbal commitment to guarantee a loan binding according to halacha?

Answer:  The halacha is that a verbal commitment is binding. 

Although we refer to such a person as a cosigner, in lashon hakodesh, we use the word areiv, guarantor. We do not refer to any signing because arvus works even verbally.

Question: Does this commitment of the guarantor need to be expressed at the time of the loan or can he say afterwards that he is now accepting to be areiv?

Answer: There is a machlokes Tannaim if one who accepts responsibility after a loan has already been given is liable as an areiv. 

The halacha follows the view that such a commitment is valid as arvus; however, there is an important distinction between arvus accepted at the time of the loan and arvus accepted later. When one becomes an areiv at the time of the loan, no kinyan is needed, whereas when one accepts to be an areiv after the loan we pasken that a kinyan is necessary to deem him liable.