Extracting json array in Snowflake without knowing the key value

You could do a recursive flatten and then exclude the keys amount and currency since those aren't what you want. This works for the scenario you've requested, but might need tweaking if you have other keys in your actual data:

WITH x AS (
    SELECT parse_json('{
    "AccountId": {
        "providerId": "12345",
        "provider": "ABCD"
    },
    "destinationAccountId": null,
    "referenceId": "ABCD1234",
    "repaymentAmounts": {
        "75e7dae4-3000-4b51-a1e2-555218d6c180": {
            "amount": 5.20,
            "currency": "GBP"
        },
        "b7b7f744-db7f-48c9-8417-985d6fe137bc": {
            "amount": 0.24,
            "currency": "GBP"
        },
        "49c69500-e9c0-433e-bd2e-cb387d1b688f": {
            "amount": 7.30,
            "currency": "GBP"
        }
    }
}') as var)
SELECT y.key
FROM x,
LATERAL FLATTEN(input=>x.var:repaymentAmounts, recursive=>True) y
WHERE y.key not in ('amount','currency');

You can also skip the recursive query and the filter, now that I look at it closer:

WITH x AS (
    SELECT parse_json('{
    "AccountId": {
        "providerId": "12345",
        "provider": "ABCD"
    },
    "destinationAccountId": null,
    "referenceId": "ABCD1234",
    "repaymentAmounts": {
        "75e7dae4-3000-4b51-a1e2-555218d6c180": {
            "amount": 5.20,
            "currency": "GBP"
        },
        "b7b7f744-db7f-48c9-8417-985d6fe137bc": {
            "amount": 0.24,
            "currency": "GBP"
        },
        "49c69500-e9c0-433e-bd2e-cb387d1b688f": {
            "amount": 7.30,
            "currency": "GBP"
        }
    }
}') as var)
SELECT y.key
FROM x,
LATERAL FLATTEN(input=>x.var:repaymentAmounts) y;