The phrase "at a fixed amount" is correct and usable in written English.
It can be used when referring to a specific, predetermined quantity or value in various contexts, such as finance or pricing.
Example: "The subscription fee is set at a fixed amount of $10 per month."
Alternatives: "for a set price" or "at a predetermined value".
The two rates are now set by law at a fixed amount above fluctuating interest rates.
Eventually, the bonus was set at a fixed amount, which has since risen to $12,000 a year.
It's just the narrative around Bitcoin – its mysterious founder, its claims to disruption, its decision to set the total issuance of Bitcoins at a fixed amount, and so on – that cause it to stand out, Bindewald claims.
The first would continue the sale of helium at a fixed amount per year for the next year or so until BLM has recouped its investment.
Deliveries and neonatal care are expensive, but Amerigroup was being paid by the state at a fixed amount per member it enrolled, regardless of illness or condition.
Pigs were individually housed and received experimental diets twice daily (0700 and 1700 h) at a fixed amount based on BW of pigs (0.09 × BW0.75 kg).
Because of the imposition of the constraint that the logit of the propensity score of matched subjects could differ by, at most, a fixed amount, it is possible that insufficient numbers of untreated subjects will be available for matching to some treated subjects.
I work full-time and the last thing I wanted was to be forced into doing a fixed amount of work at fixed times".
Defined-benefit pension plans pay a fixed amount at retirement.
7B2 samples were cross-linked at varying protein concentrations using a fixed amount of glutaraldehyde.
Initial velocities were derived from series of reaction progress curves at different substrate concentrations with a fixed amount of enzyme (36 nM) and plotted against PI(4 P concentration.
