If you had under-declared the value of your imports and under-paid GST, you need to pay the additional GST to Singapore Customs by taking up a short payment permit. You can claim the total GST amount as reflected in the original and short payment permit in your GST return.
If you had over-declared the value of your imports and over-paid GST, you can claim the GST amount as reflected in your import permit but report the correct value of imports in your GST return.
Refer to Mistakes in Import Declarations for detailed instructions on what you should do in the above situations.
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