You actually do not currently meet the financial requirement because the UK job offer is the most important part. Your current US job means nothing unless you can prove you will have at least the same income in the UK.
The letter from your current employer is irrelevant and will not count unless they can guarantee that the job will continue in the UK - if they can't guarantee this, there is no need to include a letter from them at all.
So, you can't apply for any visas until you have your future UK income sorted out - if you apply before this, the visas will be automatically refused.
The whole point of the financial requirement is to guarantee you will have an annual income of at least £24,800 in the UK AFTER you have moved.
You do this using employment income in one of 3 ways:
1) you move to the UK ahead of your family, get a job and work for 6 months, earning at least £24,800, before you can apply for their visas.
Or
2) you have a guaranteed job offer in the UK, paying at least £24,800 and starting within 3 months of moving back AND you have earned at least £24,800 in the US in the last 12 months
Or
3) you have been with your current company, earning at least £24,800 for at least 6 months AND this job will continue at the same salary after you have moved to the UK.
In short: If you can't guarantee future UK employment, you cannot apply for the visas yet.
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