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Cash and Government Bonds as Short-Sale Collateral

Article Quant Q&A · Author: actinidia
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Summary

The document examines whether cash and government bonds are interchangeable when posted as collateral for borrowing a stock. The accepted answer explains that cash collateral typically earns or pays an agreed rate, such as a rate linked to federal funds, while bond collateral requires passing through any coupons received. A borrower can also use posted bonds in the repo market to obtain cash, paying the repo rate. In this framing, the practical financing difference between cash and bonds is tied largely to the applicable cash and repo rates.

Another answer emphasizes that bonds carry market-price risk whereas cash does not: a bond’s value could fall before collateral is returned, creating a shortfall if it must be sold to repay cash. If the bonds themselves are the collateral, their return obligation differs from an obligation to return a fixed cash amount. The discussion is conceptual and does not quantify rates, haircuts, margin calls, or contract terms. Thus, the degree of fungibility depends on collateral arrangements and market conditions, including bond-price risk.

Key ideas

  • Cash collateral may accrue interest at an agreed rate, while bond collateral entails passing through coupons.
  • Bonds can be used in repo transactions to raise cash, with repo financing costs.
  • The accepted answer frames the main financing distinction as the difference between the relevant cash rate and repo rate.
  • Government bonds carry price risk, while cash does not fluctuate in market value.
  • Collateral terms, bond-price movements, and financing rates affect how closely cash and bonds can substitute for one another.

Tags

Fixed incomeMarket microstructureRisk management
Full text
# Are risk-free-rate bonds and cash fungible?


# Are risk-free-rate bonds and cash fungible?












I had a thought experiment: suppose you wanted to borrow an equity security from me (perhaps to short sell it). I ask you for collateral and a borrow fee, and in exchange you get the stock.

If you give me cash collateral, I will buy Treasuries and make the risk-free rate until you cover. In a year, you give me back the securities, and I sell the bonds to give you back the collateral, keeping the difference.

Now, what if you give me Treasuries as collateral? Is there any way to profit from this? At first I thought I'd use the Treasuries as collateral to borrow cash, but by no-arbitrage whomever I'd borrow from is charging me the risk-free rate at least, which takes my profits away.

Intuitively it seems wrong to me that RFR bonds and cash wouldn't be fungible in every material way, but I can't find an analogous yield-generating move here that would suggest otherwise.

## Answer by dm63 (score 6, accepted)

https://quant.stackexchange.com/a/72116

First of all your statement is not quite correct. If you receive cash as collateral, you have to pay me interest at whatever rate we have agreed to (probably Fed Funds). If you receive bonds as collateral, you have to pay to me any bond coupons you receive during the transaction. You can use the bonds as collateral to borrow cash in the repo market , in which case you pay the repo rate as interest. Thus, cash and bonds are almost fungible , the only difference being Fed funds versus repo rate.

## Answer by Mercadian (score 0)

https://quant.stackexchange.com/a/72090

Cash is King.

The way I see it is that they are not fungible because they have different market risk profiles, Cash is truly market risk neutral while government bonds are not.

In your scenario, what would happen if the treasuries you bought with the cash collateral are down by the time of the short cover? you'd need to sell them at a loss, come up with the missing cash and give it back.

Now, same situation but this time you received bonds as collateral, not cash... well, your collateral in case of a credit event would be degraded, but you could give it back without a loss at the cover.

In both instances you can do a collateral or margin call if the short position is souring, but nominally you need consider your obligation in terms of what needs to be returned.

Note all of this analysis does not include inflation.

hope this helps,

M

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This summary was written by Stratmill's research agent from the original; it is not a copy of the source.