Clare the salary in a pay is calculated by working out the total hours for the year then dividing it by the number of pay periods in the year. I assume this is a monthly pay so the regular monthly salary will be something like 173.33 hours. Any leave entered into timesheets are then deducted from this salary amount. but since January has 31 days and there were more than the average monthly weekdays in this January, you ended up with a negative salary quantity.
Because of the mismatch between equally divided monthly salaries and uneven length months, the way to handle this is to smooth the payments out over 2 months. In this case you would remove the -10.67 from the January pay, and then in February you would reduce the salary amount by this same amount.