While conventional deposits are generally instruments that are sifted against credit risk, there are residual credit risks. Although this is essentially a guaranteed transaction, the seller may not buy back the securities sold on the due date. In other words, the pension seller does not fulfill his obligation. Therefore, the buyer can keep the warranty and liquidate the guarantee to recover the borrowed money. However, security may have lost value since the beginning of the operation, as security is subject to market movements. To reduce this risk, deposits are often over-insured and subject to a daily market margin (i.e., if the guarantee ends in value, a margin call may be triggered to ask the borrower to reserve additional securities). Conversely, if the value of the guarantee increases, there is a credit risk to the borrower, since the lender is not allowed to resell it. If this is considered a risk, the borrower can negotiate a subsecured repot.  A pension contract is a short-term guaranteed credit: one party sells securities to another and agrees to buy them back at a higher price at a later date. The securities serve as collateral. The difference between the initial price of the securities and their redemption price is that of the interest paid on the loan called the pension rate. For the party that sells security and agrees to buy it back in the future, it is a repo; for the party at the other end of the transaction, the purchase of the warranty and the consent to sell in the future, it is a reverse buyback contract. Beginning in late 2008, the Fed and other regulators adopted new rules to address these and other concerns.
One consequence of these rules was to increase pressure on banks to maintain their safest assets, such as Treasuries. They are encouraged not to borrow them through boarding agreements. According to Bloomberg, the impact of the regulation was significant: at the end of 2008, the estimated value of the world securities borrowed was nearly $4 trillion. But since then, that number has been close to $2 trillion.