diff --git a/lib/v1/transactions/base.js b/lib/v1/transactions/base.js
index 1b88d5549..035fc8da6 100644
--- a/lib/v1/transactions/base.js
+++ b/lib/v1/transactions/base.js
@@ -191,7 +191,7 @@ module.exports = {
             additionalProperties: false,
             description: 'Cashier face values.',
             properties: {
-              number: {
+              custom_id: {
                 type: 'string',
                 example: '005'
               },
diff --git a/lib/v1/transactions/items/base.js b/lib/v1/transactions/items/base.js
index 1000939b6..702a40751 100644
--- a/lib/v1/transactions/items/base.js
+++ b/lib/v1/transactions/items/base.js
@@ -77,6 +77,7 @@ module.exports = {
     maxLength: 3
   },
   product: {
+    default: null, // in case of expenses a product makes no sense
     ...oneOf({
       type: 'string',
       format: 'uuid',
@@ -92,6 +93,15 @@ module.exports = {
       example: 'AKI-1234566'
     })
   },
+  tax: {
+    default: null,
+    ...oneOf({
+      type: 'string',
+      format: 'uuid',
+      example: '9d110eb2-2245-4e14-9574-49b502f3a9b3',
+      description: 'If a single tax applies (normally VAT) - the applicable Tillhub tax resource with tax accounts.'
+    })
+  },
   vat_rate: {
     type: 'number',
     description: 'The VAT rate of this item. E.g. 0.19 for 19% German VAT. If no VAT applies, set to 0.',
@@ -140,10 +150,12 @@ module.exports = {
               type: 'string',
               example: 'a3be1e02-7983-4cba-b7b8-94453c0536ef'
             },
-            order_index: {
+            position: {
               description: 'Position of the specific tax.',
               type: 'integer',
-              example: '0'
+              minimum: 0,
+              maximum: 30,
+              example: '1'
             },
             rate: {
               type: 'number',
@@ -154,16 +166,21 @@ module.exports = {
               multipleOf: 0.001
             },
             // flat values here because we do not want to align another array from context
-            account: {
-              description: 'Custom number identifier.',
-              type: 'string',
-              example: '440165'
+            custom_id: {
+              default: null,
+              ...oneOf({
+                description: 'Custom tax account number identifier.',
+                type: 'string',
+                example: '440165'
+              })
             },
             display: {
-              description: 'Custom description.',
               default: null,
-              type: 'string',
-              example: 'Another tax'
+              ...oneOf({
+                description: 'Custom tax account description.',
+                type: 'string',
+                example: 'Another tax'
+              })
             }
           }
         }),
@@ -292,7 +309,7 @@ module.exports = {
             type: 'object',
             description: 'Account face values.',
             properties: {
-              number: {
+              custom_id: {
                 description: 'Account number.', // account or expense_account fa_account_number
                 type: 'string',
                 example: '1776'
@@ -306,12 +323,32 @@ module.exports = {
           }),
           default: null
         },
+        tax: {
+          ...oneOf({
+            type: 'object',
+            description: 'In case of a single tax (usually VAT) - the tax account face values.',
+            properties: {
+              custom_id: {
+                description: 'Custom number identifier.',
+                type: 'string',
+                example: '440165'
+              },
+              display: {
+                description: 'Custom description.',
+                default: null,
+                type: 'string',
+                example: 'Another tax'
+              }
+            }
+          }),
+          default: null
+        },
         product: {
           ...oneOf({
             type: 'object',
             description: 'Product face values.',
             properties: {
-              number: {
+              custom_id: {
                 type: 'string',
                 example: '0032039845'
               },
@@ -328,7 +365,7 @@ module.exports = {
             type: 'object',
             description: 'Product face values.',
             properties: {
-              number: {
+              custom_id: {
                 type: 'string',
                 example: '0032039845'
               },
@@ -349,7 +386,7 @@ module.exports = {
                 type: 'boolean',
                 example: 'true'
               },
-              number: {
+              custom_id: {
                 type: 'string',
                 example: '012'
               },
@@ -369,8 +406,7 @@ module.exports = {
         }
       },
       required: [
-        'account',
-        'product'
+        'account'
       ]
     }),
     default: null
diff --git a/lib/v1/transactions/payments/base.js b/lib/v1/transactions/payments/base.js
index 7a8b8fba7..3437de8a6 100644
--- a/lib/v1/transactions/payments/base.js
+++ b/lib/v1/transactions/payments/base.js
@@ -48,10 +48,10 @@ module.exports = {
     default: null,
     ...oneOf({
       type: 'number',
-      description: 'The position description of the payment item. E.g. positon 1 (2, 3 etc.) in the cart.',
-      example: '1',
-      maximum: 32767,
-      minimum: 0
+      description: 'The position of the payment item. E.g. positon 1 (2, 3 etc.) in the transaction.',
+      minimum: 0,
+      maximum: 300,
+      example: '1'
     })
   },
   date: {
@@ -190,7 +190,7 @@ module.exports = {
             additionalProperties: false,
             description: 'Account face values.',
             properties: {
-              number: {
+              custom_id: {
                 type: 'string',
                 example: '1009'
               }
@@ -211,10 +211,7 @@ module.exports = {
           }),
           default: null
         }
-      },
-      required: [
-        'account'
-      ]
+      }
     }),
     default: null
   },