When creating a sales position for quantitative items via the API endpoint warehouse/sale-items/create, it is mandatory to specify the purchase position identifier "purchaseItemId" — a reference to the purchase document line from which the item quantity will be deducted. If this field is not provided for a quantitative item, the system returns the error: "The item is not a service, therefore the item sales position must have a reference to the item position in the purchase document." For services, this field is not required — it is mandatory only for quantitative items (when "isQuantitative" = true). Note. This error is only relevant when creating sales documents via the API. When a sales document is created in the Site.pro interface, the system selects the required purchase position automatically. Sales position fields Field requirements are described in more detail in the API documentation. Field Type DescriptionsaleIdNumberSales document ID.itemIdNumberItem ID.warehouseIdNumberWarehouse ID.quantityNumberQuantity.priceWithoutVatNumberPrice excluding VAT.vatRateNumberVAT rate (e.g., 21).priceWithVatNumberPrice including VAT.vatClassificationIdNumberVAT classification ID.purchaseItemIdNumberPurchase position ID, obtained via warehouse/stock-list. Mandatory for quantitative items. How to get the purchase position identifier You do not need to look for the purchase position ID in the item register (reference-book/items/list) — items available for sale and their balances are provided in the stock balance API endpoint warehouse/stock-list. In the request, specify the sales document ID and the item: {"saleId": 388,"filters": {"groupOp": "AND","rules": [{"field": "itemId", "op": "eq", "data": 4221}]}} In the response, find the required balance record and retrieve its purchase position identifier. If you want to deduct from the oldest purchase, filter the balances by item, warehouse, and purchase date. Use the retrieved value in the "purchaseItemId" field when creating the sales position: {"calculationMode": 6,"saleId": 388,"itemId": 4221,"warehouseId": 1,"quantity": 1,"priceWithoutVat": 100,"vatRate": 21,"priceWithVat": 121,"vatClassificationId": 1,"purchaseItemId": 595} Important. The date of the sales document cannot be earlier than the item's purchase date — you cannot sell an item that was not yet in the warehouse at that time. If the balance request returns an empty result, first check the purchase and sales dates. Note. Neither the purchase document ID nor the item ID is suitable for the \ Note. If the quantity being sold is greater than the balance of a single purchase position, split the sale into multiple sales positions — specify the corresponding purchase position and quantity for each. Note. Different API methods are used to search for items in purchase and sales documents: for purchases – “Item list” https://site.pro/My-Accounting/doc/api#api-warehouse-stock-list,, for sales – “Warehouse item balances” https://site.pro/My-Accounting/doc/api#api-reference-book-items-list. Related articles: Where can a programmer find the warehouse ID?How to use the bundle/debundle API endpoint?