Freelancers and independent contractors have the right to set their own rates. You can contract them for a job based on the approved budget, but they still have to agree to that rate, based on the hours they expect to spend to execute the assignment. As for “being on standby,” freelancers are by the nature of their jobs always on call. That said, if you need to verify a freelancer’s availability, it is judicious to pay him/her.
Standby essentially means “right of first refusal.”
“We’d like you to be available on [date], but will have to confirm. Can you be on standby? Call us if something else comes up, and we will release you from standby, or confirm we need you.”
If you work with this freelancer regularly, rather than on occasional projects, you might consider negotiating a monthly fee/retainer, eliminating the need for a special “standby rate.” The retainer ensures the freelancer will be “available” to you for a specified number of hours each month.
Under a retainer arrangement, the freelancer would roll over unused hours to the following month, just as your agency would do if under retainer to a client, and you did not actually use the allotted hours. As with an agency retainer, pay the freelancer in advance for the month to come, then do an end-of-month recap to see if there are carryover hours or overuse that needs to be balanced.
Other than a retainer, you could simply contract the freelancer to be available by paying a 10-15% deposit on the total project estimate as a “stand-by fee.” Let the freelancer know if the project does not come to fruition, they can keep that as a cancellation or “kill” fee. Under this arrangement, ask the freelancer not to accept other work for the designated dates without checking with you first.
Whatever arrangement you make, put it in writing, with signatures.
As for having the freelancer “fill-in” for an absent full-time person, I would contract the freelancer for the designated period. Again, they have the right to set their hourly rate. But with the guarantee of a few weeks of steady work, the freelancer may be amenable to negotiating a “blended rate” based on the freelancer doing a mix of admin, design, copy, production, etc., or a reduced “day rate” based on the guaranteed hours. Ask the freelancer what they would prefer, and negotiate their suggested payment method if necessary. Let us know how this works out: firstname.lastname@example.org