The cap
One week’s rent, worked out from the rent in your advert (or the tenancy or property rent when there is no advert). A holding deposit is not a way to take a larger sum early; the tenancy deposit rules govern that separately.
What it is for
It takes the property off the market while referencing runs. That is the whole purpose, and it is why the rules on keeping it are narrow.
Repaying it
It normally has to be repaid or put towards the rent or deposit within a short, fixed window. There are limited grounds on which it can be kept, and "I changed my mind about them" is not one of them.
When you apply it, say what it went towards, the first rent or the tenancy deposit. When you return it, enter the day the money went back. Both are kept on the record and in your activity log.
Taking it by card
In your own account, a holding deposit you ask for can be paid by card. Under Ask the applicant, tick Let them pay by card, and the applicant pays on their link, through Stripe, straight into your own Stripe account. It needs your payout account set up in Settings, and it comes with the 3-day trial and every paid plan.
The deadline for agreement is counted from the day the payment goes through, and the applicant is emailed a receipt. The link takes no more than one week’s rent. Stripe’s card fee comes out of your Stripe balance: you cannot charge the applicant a fee for paying by card, and Stripe does not give its fee back on a refund.
To give it back, press Return on the deposit and choose the refund to their card, which is paid from your Stripe balance. If your available Stripe balance does not cover it, Stripe holds the refund until it does, or fails it if that takes too long, and the deposit shows as not repaid yet. The day the refund goes through is recorded as the repayment date, and the applicant is emailed then. If a card refund fails, repay it another way, such as a bank transfer, and record the day with Record the repayment.
A holding deposit on an agency’s property cannot be paid by card, because the agency would be holding the money for someone else.
Why record it
Because the argument, when there is one, is about what was taken, when, and what was said at the time. A dated record settles it in a sentence.