The phrase "an extra amount" is correct and usable in written English.
It can be used when referring to an additional quantity or degree of something, often in contexts involving measurements, costs, or resources.
Example: "The project will require an extra amount of funding to cover unforeseen expenses."
Alternatives: "an additional quantity" or "a surplus amount".
Its complicated and sophisticated inner life requires an extra amount of the listener's attention.
Supposedly, they put an extra amount of this chemical in there, and it never killed the deal. . . .
As for Quixotic types, that required an extra amount of dedication, with loneliness and ignominy the likely pay-off.
On the other hand, the inlaid NPs give an extra amount of space for the dye-uptake.
A simpler way of putting it is that, in return for a lump sum payment, you'll get an extra amount of pension per week.
During the mortgage bubble, some lenders kept monthly loan payments low by not tacking on an extra amount to cover taxes and insurance.
By a conservative estimate, current concentrations are trapping an extra amount of energy equivalent to 400,000 Hiroshima bombs exploding across the face of the earth every day.
Combined, that could make it a systemically important bank, requiring an extra amount of capital that would increase the cost of doing business.
When the pension fund's short-term investment returns fall short of the long-term projection, then the employer contributes an extra amount in order to make up the difference.
Because he is not participating in the campaign finance system, the spending cap will also be lifted for the Democratic nominee, who will get an extra amount of city matching money.
Between 60 and 120 metres away, it's proposed that houses will be in a voluntary purchase zone, so HS2 Ltd will buy any property at a supposed market price, plus an extra amount to cover costs.
