Hi, Please refer the following link https://apps.odoo.com/apps/modules/10.0/skit_step_discount/ it will allow discount on each product both in POS and sales order. Hope this will help you

After installing and testing the addon, i am facing a bug.

I can not export the invoice in pdf.

I have this error message :

Erreur:
Odoo Server Error

Traceback (most recent call last):
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\qweb.py", line 344, in _compiled_fn
    return compiled(self, append, new, options, log)
  File "<template>", line 1, in template_account_report_invoice_document_with_payments_4445
  File "<template>", line 2, in body_call_content_4444
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\ir_qweb.py", line 387, in _get_field
    content = converter.record_to_html(record, field_name, field_options)
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\ir_qweb_fields.py", line 454, in record_to_html
    return super(MonetaryConverter, self).record_to_html(record, field_name, options)
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\ir_qweb_fields.py", line 126, in record_to_html
    return False if value is False else record.env[self._name].value_to_html(value, options=options)
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\ir_qweb_fields.py", line 412, in value_to_html
    fmt = "%.{0}f".format(display_currency.decimal_places)
AttributeError: 'str' object has no attribute 'decimal_places'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\web\controllers\main.py", line 1673, in report_download
    response = self.report_routes(reportname, docids=docids, converter=converter)
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\http.py", line 517, in response_wrap
    response = f(*args, **kw)
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\web\controllers\main.py", line 1614, in report_routes
    pdf = report.with_context(context).render_qweb_pdf(docids, data=data)[0]
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\ir_actions_report.py", line 677, in render_qweb_pdf
    html = self.with_context(context).render_qweb_html(res_ids, data=data)[0]
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\ir_actions_report.py", line 711, in render_qweb_html
    return self.render_template(self.report_name, data), 'html'
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\ir_actions_report.py", line 501, in render_template
    return view_obj.render_template(template, values)
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\ir_ui_view.py", line 1276, in render_template
    return self.browse(self.get_view_id(template)).render(values, engine)
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\web_editor\models\ir_ui_view.py", line 27, in render
    return super(IrUiView, self).render(values=values, engine=engine, minimal_qcontext=minimal_qcontext)
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\ir_ui_view.py", line 1285, in render
    return self.env[engine].render(self.id, qcontext)
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\ir_qweb.py", line 59, in render
    result = super(IrQWeb, self).render(id_or_xml_id, values=values, **context)
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\qweb.py", line 272, in render
    self.compile(template, options)(self, body.append, values or {})
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\qweb.py", line 346, in _compiled_fn
    raise e
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\qweb.py", line 344, in _compiled_fn
    return compiled(self, append, new, options, log)
  File "<template>", line 1, in template_554_4393
  File "<template>", line 2, in body_call_content_4392
  File "<template>", line 3, in foreach_4391
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\qweb.py", line 351, in _compiled_fn
    raise QWebException("Error to render compiling AST", e, path, node and etree.tostring(node[0], encoding='unicode'), name)
odoo.addons.base.models.qweb.QWebException: 'str' object has no attribute 'decimal_places'
Traceback (most recent call last):
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\qweb.py", line 344, in _compiled_fn
    return compiled(self, append, new, options, log)
  File "<template>", line 1, in template_account_report_invoice_document_with_payments_4445
  File "<template>", line 2, in body_call_content_4444
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\ir_qweb.py", line 387, in _get_field
    content = converter.record_to_html(record, field_name, field_options)
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\ir_qweb_fields.py", line 454, in record_to_html
    return super(MonetaryConverter, self).record_to_html(record, field_name, options)
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\ir_qweb_fields.py", line 126, in record_to_html
    return False if value is False else record.env[self._name].value_to_html(value, options=options)
  File "C:\Program Files (x86)\Odoo 12.0\server\odoo\addons\base\models\ir_qweb_fields.py", line 412, in value_to_html
    fmt = "%.{0}f".format(display_currency.decimal_places)
AttributeError: 'str' object has no attribute 'decimal_places'

Error to render compiling AST
AttributeError: 'str' object has no attribute 'decimal_places'
Template: account.report_invoice_document_with_payments
Path: /templates/t/t/div/div[2]/div/div/table/tr[2]/td[2]/span
Node: <span t-field="o.amount_discount" t-options="{&quot;widget&quot;: &quot;monetary&quot;, &quot;display_currency&quot;: &quot;o.currency_id&quot;}"/>

Any idea ?

Use the pricelist feature!
https://www.odoo.com/documentation/user/12.0/sales/products_prices/prices/pricing.html

BR

Ricardo