Product: Student loan

I am on income based repayment for my student loans that are now being handled by Navient. On  XXXX / XXXX /  XXXX   I submitted my required monthly payment online. I then called Navient to make an additional payment of {$570.00} that I wanted applied to just the principal of a particular loan.  XXXX , the agent who took my call, informed me that I could not make a payment to my principal unless I did it online at the same time I was making my required monthly payment. When I inquired as to why, I was told that " interest accrues daily, '' and if I " made a payment a few days ago, the loan amount would n't be the same because interest has been accruing since the minimum monthly payment was made. Therefore, a payment to the principal could not be made until the following month when the required minimal monthly payment was made. '' I informed him that I made my payment today,  XXXX , and I want to make an additional payment, now towards the principal, on the same day,  XXXX . The agent reluctantly looked into that particular loan so that I would make a payment to my principal. Then I was told that I can not make a payment on my principal because there is outstanding interest on that loan. I told him that I did n't think it was fair that I can not put money towards the principal when I already paid my required payment, which was being applied to the interest. He refused to put my payment towards my principal, so I did not make an additional payment at that time.     I asked to be sent in writing a letter that states that I can not make a payment to my principal if I have outstanding interest on my loan, even though I already made my required monthly payment. He said he would send me a breakdown of how my payments were applied. I said No, that I wanted my specific question addressed in this letter, not a generic breakdown of how my payments are applied to my loan. He said it that my question would be answered, and it would come via mail, most likely after the  XXXX  of  XXXX .     After the first year of being on income based repayment, my total loan amount increased by {$1000.00}. My payments on income based repayment do not even cover the interest on my loans, and I will therefore never be able to pay off this debt if I can not make additional payments applied only to the principal.     I felt that  Navient  was trying to give me excuses as to why I can not make additional payments so that interest will keep accruing and the company will make even more money off of my loans. I do not think this is fair or ethical ( and possibly not legal - as I am still researching this matter. ) I felt I was initially lied to or attempted to be purposely mislead into thinking that I can only make payments to the principal of my loans online at the same time as making the minimum monthly payment.     The student loan repayment website is also misleading. The section that allows you to make additional payments only has the option of keeping the following due date the same or advancing it to a later date. My auto loan, for example, has a section that clearly states " optional additional payment to principal only. ''  Navient  makes it intentionally difficult to make payments to the loan principal and essentially pay off student loans.     I also felt that when I asked for it to be stated in writing " that I legally can not pay off my principal if there is any outstanding interest on a loan, even after making the required monthly payment, '' the agent tried to give me the run around and tell me that other information skirting around my question would be sent to me, and that Navient is trying to avoid giving me direct written answers to my questions.