The Rebate Rate and Negative Rebates

When a short seller borrows shares, they don't hand the lender an IOU — they sell the shares and post the cash proceeds with the lender as collateral against the loan. The lender invests that cash and pays some of the interest back to the short seller; that payback is called the rebate rate. For an ordinary, easy-to-borrow stock, the rebate sits close to the prevailing short-term interest rate, so borrowing is nearly free — the short seller gives up only a small spread versus what they'd earn holding the cash themselves.

For a genuinely scarce stock, the lender has pricing power and keeps more of that interest, quoting a rebate below the general market rate. Push scarcity far enough and the rebate can go negative: instead of the lender paying the short seller interest on the collateral, the short seller pays the lender on top of forfeiting all the interest the cash would normally earn. A negative rebate is, functionally, an explicit fee layered on top of the lost interest income — the market's way of pricing a borrow that has no spare supply at any positive rebate.

Worked example

Short-term interest rates are running at 5%. Compare two names, both requiring a $1,000,000 short position financed by posting $1,000,000 cash collateral for one year:

	General collateral	Deeply scarce name
Market interest rate	5%	5%
Rebate rate paid to short seller	4.8%	−3%
Interest earned/(paid) on collateral	+$48,000	−$30,000
Implicit annual cost vs. risk-free cash	$2,000	$80,000

For the general collateral name, the short seller gives up only $2,000 of the $50,000 they'd have earned holding cash outright — an almost negligible drag. For the scarce name, the negative rebate means the short seller actively pays out $30,000 on top of losing the $50,000 they would have earned — an $80,000 total swing versus holding cash, on a trade with no other explicit "fee" line item anywhere in the paperwork.