Business Central doesn't really have anything out of the box for invoice retentions without customizing the system or getting an add-on that does it.
You could try to use invoice prepayments, on an order you would create a prepayment invoice for 5%. This would essentially be your retention agreement/invoice. Then you would post the final invoice for the remaining 95%. This way from 2 orders you would have 2 invoices, 1 for 5% and 1 for 95%.
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