May one forgive a loan without telling the borrower?
If one loaned someone money and has yet to be paid back, often the lender who sympathizes with the borrower’s predicament, desires to simply forgive him the loan. The question however is raised whether the borrower must be made aware that his loan has been forgiven and he thus no longer needs to repay it. This touches on two Halachic aspects: 1) Since a borrower is required to repay a loan, perhaps he transgresses not repaying it so long as he is not made aware that the loan has been absolved, even if in truth it has been. 2) Perhaps in order for the pardoning of the loan to be legally valid, it must be done with the knowledge of the borrower. The following is the final ruling on this subject.
B. The law:
It is proper for one who forgives a loan to tell the borrower that the loan is forgiven, in order to prevent him from transgressing any sin. Furthermore, some Poskim rule that forgiving a loan is only legally binding if done with the knowledge of the borrower.
 Pela Yoeitz Halvah p. 50a “It does not suffice to forgive him the heart, as nevertheless the borrower is doing a sin, being that he is unaware that the loan was forgiven. Rather he must tell him not to worry that he can’t pay back the loan, and I forgive it.”
The reason: As a borrower who does not pay back a loan, transgresses a prohibition, as brought in Admur Halva 4; Michaber C. M. 97:3. Now, the Mishneh in Nazir 23a [brought in Rashi Matos 30:6] states that if a wife made a vow and her husband abolished it without her knowledge, and she transgresses the vow, then she needs atonement [“Vahashem Yislach Lah”]. From here we learn that when one thinks he is doing a sin in his mind it is considered that he has transgressed even if in truth he did not transgress. Thus, if one intended to eat Treif and ate Kosher, he needs atonement. [Rebbe Akiva in Nazir ibid; See Minchas Shlomo 35 regarding Yichud] The same applies in Mitzvos of Bein Adam Lechaveiro, such as if a father is Mochel on his Kavod but does not tell his son, the son still needs atonement if he does not respect him. [Tosafos Kiddushin 32a; Ritva Kiddushin ibid; Chofetz Chaim Lashon Hara 4 regarding saying Lashon Hara Letoeles; However, see Sefer Chassidim 152 who allows a father to be Mochel without telling the son; See Or Hachaim Vayigash] This is aside for the fact that there are Poskim who rule a loan is not forgiven unless it is done in front of the borrower.
 See Erech Hashulchan Choshen Mishpat 12:5; Divrei Hageonim 57:1; Pischeiy Teshuvah C.M. 241:1 in name of Beis Meir E.H. 35; However, see Yabia Omer 3:2-5