The collar would be tied to the $/BTC exchange rate, and the best way to illustrate is by example:
Lets say you lend me 500 BTC, and at funds transfer the exchange rate is $10/BTC.
For example sake, lets assume you have earned 50 BTC in interest, and now want to call all 550 BTC. However, the exchange rate is now $4/BTC. Rather than getting back only your 550 BTC worth $2,200 (when you lent 500 BTC worth $5,000!), we would use a formula to ensure the 30% collar is dollar terms. In this case we would take $3,850 (550*7) / 4 (spot rate) and arrive at a repayment of 962.50 BTC.
Essentially, we are insulating both borrower and lender against extreme loss of purchasing power by putting in a floor and ceiling on the exhange rate. However, the collar only kicks in once the exchange rate has moved beyond the collar limits.
I fully understand the example, but I can't describe it. Maybe, "The purchasing power of the returned asset is limited to a gain or loss of 30% by varying the amount returned"?
That would probably fit!
I use "price collar" loosely.....but it is a good approximation.
This. Exactly this.