An amount equal to
Towards more picturesque speech™

Excellent celery that will fill out any drafting that looks a bit thin. It may look like — and is, truth be told — a pointless nominalisation^{[1]}, but this will not stop assiduous attorneys defending it with their miserable little lives:
 “...on each Payment Date the Floating Amount shall, unless otherwise agreed, be reduced by an amount equal to such rate as the Calculation Agent and/or Determination Agent, as the case may be, shall calculate or determine, as the case may be, to be the Floating Rate Payer’s unadjusted cost of funding.”
So, why add “an amount equal to”?
Profound ontological uncertainty — something in which, like anal retentiveness, derivatives practitioners are uniformly steeped — is why. For, they will say, it isn’t actually the borrower’s cost of funding you’re deducting — that is not a number but a (different) incorporeal, abstract concept which happens to resemble a number but is not one, and is therefore logically incapable of being deducted from something else by mathematical operation.
Thus, a neat solution: add the phrase “an amount equal to”, which “monetises” this concept, turning it into something properly numeric^{[2]} that a fellow can articulate in pounds, shillings and pence.
Except that there’s a flaw in this logic: “equals” is a mathematical operator as surely as is an addition or a subtraction. It is no more (or less) susceptible to manipulation by ethereal concepts. If you can’t deduct a cost of funding from a rate, you can’t calculate an amount equal to it, either.
But of course you can deduct a cost of funding. Thus the forensic value of the expression “an amount equal to” is an amount equal to nothing.