Unfortunately, under the FIDIC Red and Yellow Books 1999, the right of an Employer to set off from an amount already certified in a Payment Certificate but unpaid is inexplicit.
Once the Employer has a Sub-Clause 3.5 determination, it may ask the Engineer to deduct the amount determined from the next Payment Certificate. This is clear.
But rather than rely on the Engineer, can the Employer instead, itself, deduct by way of set off from an amount already certified in a Payment Certificate but unpaid? This is not clear.