I think you’re starting to follow the logic, but you’re making it too complicated. In a futures contract, each side is required to post the initial margin. The initial margin for futures is standardized based on the underlying. It’s not like the initial margin posted in a stock trading account.
The benfit (or flaw) with futures is that the initial margin is a fraction of the contract value, giving you an opportunity to access the futures market with high levels of leverage. So, in your example, a futures contract worth $10,000 would most likely require only a small amount of margin to be posted. To keep the concept easy, we’ll say that the short and the long both deposit cash.
This cash is a good faith deposit that is required by the exchange to engage in the contract. While I’m having difficulty linking your original example of a mortgage, I suppose you could consider this initial margin an escrow account. Much like escrow, this initial margin can earn interest, leading to money earned on the initial margin, or collateral, hence collateral yield.
An example: Crude oil expiring in December is trading at ~$76. A crude oil contract is 1,000 barrels. $76 multiplied by 1,000 barrels is…. $76,000 worth of oil. The initial margin is around $3,700. So, for $3,700 collateral or initial margin, you control $76,000. You can earn interest in the $37,000. Assuming the contract is held to expiration (which would be silly unless you want to take delivery of the oil), the long pays the remaining value, and the short delivers 1,000 barrels.