If he is salaried and his lowest payslip is £1,400, his annual income for the visa will be calculated as:
£1,400 x 12 = £16,800
As this is less than £18,600, unfortunately, it means you won't qualify for the visa.
However, if he is non-salaried (paid by the hour) they will add up all the payslips from the 6 months, divide by 6 and multiply by 12 to get the total annual income.
The P60 amount is not relevant because he has to show the last 6 months of earnings have been £1,550 per month or more for salaried employment (or a total of £9,300 for non-salaried) and the P60 only shows total earnings for the last 12 months.
If you had applied using Category B and provided 12 months of payslips and bank statements showing the total earnings of £20,000 over the whole year, you would meet the requirements, but with Category A, salaried employment, only that £1,400 payslip will be considered.
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