While classic repos are generally credit-risk mitigated instruments, there are residual credit risks. Though it is essentially a collateralized transaction, the seller may fail to repurchase the securities sold, at the maturity date. In other words, the repo seller defaults on their obligation. Consequently, the buyer may keep the security, and liquidate the security to recover the cash lent. The security, however, may have lost value since the outset of the transaction, as the security is subject to market movements. To mitigate this risk, repos often are over-collateralized as well as being subject to daily mark-to-market margining (i.e., if the collateral falls in value, a margin call can be triggered asking the borrower to post extra securities). Conversely, if the value of the security rises there is a credit risk for the borrower in that the creditor may not sell them back. If this is considered to be a risk, then the borrower may negotiate a repo which is under-collateralized.