Posting Down Payments
You cannot post a down payment with reference to more than one down payment request.
Posting Multiple Payments for One Invoice
If a customer pays an invoice in several payments, you post the first payment with reference to the invoice, and post the amount outstanding as a residual payment. You post subsequent payments as partial payments.

Restrictions for posting with deferred tax:
1. You cannot post documents with deferred tax codes that you cannot clearly assign to one of the document types from the list of permitted document types. The system issues an error message. This concerns all types of combined documents such as down payment clearings and invoices that are posted in one document.
2. You cannot assign payments to residual items of a preceding payment to the original invoice. Post a partial payment instead of a payment with residual items.
3. You cannot post documents with deferred tax that contain customer line items and vendor line items. The system issues an error message when you enter the documents.
4. You must not assign down payment clearings to more than one down payment and one invoice.
5. You may only clear down payments completely with an invoice.
6. You cannot process documents with a foreign currency that is not the local currency.
7. You cannot process documents with more than one tax code.
8. You cannot process cross-company code documents with deferred tax.
9. You cannot process postings with tax codes with non-deductible input tax.
10. You cannot process payments with open items that have the same debit/credit indicator as the open items of the invoices or credit memos that they refer to.
11. You cannot process invoice documents with withholding tax lines.
12. You cannot process documents with tax jurisdiction codes.
13. Payments that refer to a payment but not to an invoice are not updated in the DEFTAX_ITEM table. As a result, you cannot process these payments.

Прошу прощения, но п.2 это 'residual items of a preceding payment to the original invoice', а я делаю первый платёж

Post a partial payment instead of a payment with residual items.