The date of supply rule
UAE VAT is due on the date of supply. For most supplies of goods and services, the VAT Decree-Law sets that date as the earliest of a few events: the goods being transferred or the service completed, the receipt of payment, or the issue of a tax invoice. Whichever happens first fixes the tax period, to the extent of the payment or invoice.
So if a customer pays in advance, the date of supply for that amount is the date you receive it. VAT on the deposit belongs in the return for that period, not the period when the work is finished.